Try doing things the opposite. Instead of how do you choose your melodic synth relative to your bassline the question can be how do you choose your bassline relative to your melodic synth?
The reason why I say that is because I choose my bassline AFTER I write my melodic material. The way I choose my bassline is simple cause the majority of the time it's just a simple sine wave and note wise it just follows the root note of the harmony.
I divide my bass layers like this.
1. Subbass. (Sine Wave)
2. Bass (Sine wave)
3. Upper Bass (anything)
The reason why I use a sine wave for my two bottom layers is because for the most part every sound wave below 200hz pretty much sounds the same when you filter out all the high end. The upper bass is where all the variation comes from. Basically to me it's the high end of a bass sound (200hz+) that gives it a unique characteristic. Everything else below that just sounds the same.
how do you pick a melodic sound / chord sounds based off that?
I don't usually write my bassline first but if I were to do so then the answer would just be experimentation. There are so many different combinations of instrumentations that you can use. (Cello, Piano, Guitar) for example or (saw pluck, detuned square pad, sine bass). For the most part the two bottom layers of a bass sound (below 200hz) blend well with just about anything. Start with just using those two layers to fill out the low frequencies and then choose your lead sounds. Once you got things sounding nice then maybe you can go back and experiment with the upper bass layer.
do you re synthesize the sound so its basically the same sound as the bassline with slightly different qualities so it sounds concise?
Sometimes I do actually. When you use the same sound but in a higher octave register it's almost like it comes from the same instrument family because they share the same characteristics. (Cellos to Violins) Naturally things blend pretty well.
I wouldn't get too stuck on that though cause you don't need to use the same sound in the bass as you do in the leads. Anything goes. Just experiment with different instrumentations until you find a combination that you really dig and make it part of your signature style. Pretty soon people will be on youtube with HOWTO videos in massive on how to make a "r3mington lead stab"
so youre saying you have 3 bass synths playing at a time though? why is this.
I know this wasn't the original question but I understood your answer but of course that brought this question up
thanks