You're dealing with a frequency question here.

Sub is only used on a system that actually can replicate those frequencies at a level that is audible. Therefore, sub bass is rather dangerous and have to be maintained other wise it could fuck your shit up and collapse your chest cavity.

But that aside, it's just a layer man. you layer in your sub so that it does have that feeling in that region. Your bass synth can easily play what the sub is doing and both of those elements combined will create a nice phat sound. Because you have 1, and audible bass synth and 2, a sub sonic feeling which grants you more power.

Depending on the style you're looking towards, your Bass synth actually become the main focus and your sub bass is an accent. But in any case, the sub is always an accent, unless it's the only bass component that you have (most times it's not because you need special replication to recreate it. but if you're not concerned about having it "sound good any where and on any thing" then don't worry about it).

On your width question, bass synths can have stereo effects on them, but just make sure that you control it appropriately. It's the sub sonics that don't register well in the stereo realm, which are almost (and probably) always left mono.

It's about the control. I suggest some research, or a conversation topic with a professional/student of the craft! also maybe more private conversations from the forums here.

so you wrote your bassline, added a synth, sylenth or massive or whatever,

you need a sub underneath all that so it sounds nice and good for the ears, basically you just need to duplicate the same notes to your sub channel, you need to eq it with a high cut at around 300hz i think, you must have compression and sidechaining

this is the most basic technique, there are more advanced techniques where you can start mixing things up with the notes...with amazing effect, i've heard it a lot in the electro house stuff..

if you are adding sub for your leads, you can put the sub bass notes at the 2nd lowest note of your leads,
say your leads start at G1, then layered into G2, then put the sub bass notes on your G2

You have your sub go up to 300hz? Shit. I cut around 150 and layer a mid level bass from 150 to 2k.

it is also important to understand that all 'Sub' frequencies are Sine Waves. Sine waves hold no harmonic content and all Low frequency information are sine waves. So why is that important? Well, lets assume that you are putting in sounds which occupy your sub frequencies, for example A sub bass (simple sine wave played at a low octave), Your kick, Your Tom percussion and you are also leaving in some sub frequency information on your bass sound. This is a really quick way to muddy up your low end, especially if you are tuning things and playing things on the same notes. Why? Because all you are doing is lining up sine waves and stacking them on top of each other. So a good way to keep your low end cleaner is to compensate the space. I usually end up having my sub frequencies ( 150hz / 170hz / even up to 300hz - depends how it sounds) occupied by one simple Sine Wave, every other sound, including my main bass will be high passed. (every other musical sound- not the kick obviously). The point is that your sub frequencies can very much be filled up by your kick drum and your sine wave bass. If your normal bass is losing to much punch, then get rid of the sine wave bass and use the sub frequencies from your main bass sound. Of course there are exceptions and keeping a lot of sub info can work sometimes but the fact is that you are just stacking up those sine waves down the bottom end.

for my sine wave bass I generally sum that to mono, couple it with my main bass sound, send them both to a buss where I will compress them or sidechain them. For the main bass I will either EQ out the entire frequency range where my sine wave bass is occupying or high pass it out to a point where the Sine wave and main bass don't seem to be cluttering each other. This can be hard in bedrooms because generally a low frequency waveform hasn't had chance to fully propagate before being reflected back at you. I guess its a case of trial and error and referencing tracks. The idea behind summing the lower frequencies to mono is to combat phase issues -  which are also a problem if you are stacking up those sub bass sine waves by having to much information down there.

Very insightful feedback! Thank you guys. A few more questions came up to mind when reading through your replies.

1) When cutting out the appropriate frequencies, is it better to use Low cuts/High cuts or Low shelfs/high shelfs? Part of me thinks that using cuts would get me a more clear sound because none of the frequencies would "spill" out. The other half of me thinks it's better to shelf the frequencies and allow it to "spill" out a little more gently so the overall song could be more full.

(This is confusing to me because I don't want my bass synth sub frequency to collide with my sub frequency, so using a low cut seems more appropriate)

2) Do you also cut out the frequencies of the kick within the sub and bass, or is sidechaining it good enough?

Very insightful feedback! Thank you guys. A few more questions came up to mind when reading through your replies.

1) When cutting out the appropriate frequencies, is it better to use Low cuts/High cuts or Low shelfs/high shelfs? Part of me thinks that using cuts would get me a more clear sound because none of the frequencies would "spill" out. The other half of me thinks it's better to shelf the frequencies and allow it to "spill" out a little more gently so the overall song could be more full.

(This is confusing to me because I don't want my bass synth sub frequency to collide with my sub frequency, so using a low cut seems more appropriate)

2) Do you also cut out the frequencies of the kick within the sub and bass, or is sidechaining it good enough?

You have to find the Q or bandwidth that fits for the mix, or one that you typically like. That's a question of taste that you will have to find out for yourself.

You can't have an absolutes, and you have to deal with it. that's why studying the frequency charts can get you there. It also helps you understand what may need to be in a certain spot of a given eq range.

I don't use sidechaining, because it's sort of lazy. Though i won't knock it's purpose, i just choose not to use it.

Sidechaining is an additive, and not a replacement for good eq practice.

Very insightful feedback! Thank you guys. A few more questions came up to mind when reading through your replies.

1) When cutting out the appropriate frequencies, is it better to use Low cuts/High cuts or Low shelfs/high shelfs? Part of me thinks that using cuts would get me a more clear sound because none of the frequencies would "spill" out. The other half of me thinks it's better to shelf the frequencies and allow it to "spill" out a little more gently so the overall song could be more full.

(This is confusing to me because I don't want my bass synth sub frequency to collide with my sub frequency, so using a low cut seems more appropriate)

2) Do you also cut out the frequencies of the kick within the sub and bass, or is sidechaining it good enough?

i use high cuts on the sub. Lowcut for everything and for mid-basses I mostly use an high shelf to take out the highs.

Very insightful feedback! Thank you guys. A few more questions came up to mind when reading through your replies.

1) When cutting out the appropriate frequencies, is it better to use Low cuts/High cuts or Low shelfs/high shelfs? Part of me thinks that using cuts would get me a more clear sound because none of the frequencies would "spill" out. The other half of me thinks it's better to shelf the frequencies and allow it to "spill" out a little more gently so the overall song could be more full.

(This is confusing to me because I don't want my bass synth sub frequency to collide with my sub frequency, so using a low cut seems more appropriate)

2) Do you also cut out the frequencies of the kick within the sub and bass, or is sidechaining it good enough?

1) you basically need to trust your ears the best you can here without letting it get to crowded. Personally I compensate a lot. If I have a Sinewave sub bass and a bass sound I am generally brutal with the EQ's. on my main bass I will High pass quite high and keep the low frequencies occupied by my sub and kick -  then buss the basses together and compress them.  YOu should also be aware that most sounds with heavy low frequency info will have some sort of Low end resonance -  Grab an EQ and make a large but narrow boost and sweep across the sub frequency range very slowly - you will notice at some point in the soun (usually between 80-100hz ish) there will be a really resonant and unpleasant frequency. I like to tame these slightly with a narrow cut on the EQ. just helps tidy it up a bit. I also like to use mid / side EQ - Taking out the side information to around 90hz ish and highpassing the mids to whatever feels right for the track.

2) -  you know I used to always EQ my kick but now I never do! ive pretty much acquired a small bank of kicks that  I like to use and they have either been processed or I do not feel the need to process them. Sometimes id maybe do a low cut at 25 -30 HZ - sometimes look for that resonant low end problematic frequency or sometimes I just leave the sample as is.  I tend to use KICK by sonic academy and sometimes it can be quite weighty on the bottom end so I go in and tame it slightly with the EQ. Its all about how you think it sounds. If you feel the sub is clouding the kick....just sidechain, get your volumes right or even just highpass the sub around 10hz abouve where the kick is peaking and see how that sounds.