How does this "everything under 150Hz has to be mono" works, I mean, how do you turn something that has a very wide stereo image into mono?, the kick and the bass for example; how do you know those low frequencies are in mono? and how do you make them mono if they are not?, and what are the benefits for having them that way.
Maybe I'm asking a lot of things but this particular issue is having me troubled and intrigued.
To turn a sound completely into mono, just turn your mixer track into a mono track. In FL it works like this (stereo knob turned all the way to the right = mono):
http://imgur.com/iWuAjszIf you're not using FL, you'll have to consult your program's manual to find out how to do it there.
To only turn the lowend of a certain sound to mono, you can use Pro-Q2's mid/side EQ like this:
http://imgur.com/II8FOxi What it does is it cuts away the side signal of the sound with a "low cut" filter. In very basic terms, the mid signal is whats equal in both the left and right channels (mono), and the side signal is what's different in both channels (stereo).
In my opinion, you shouldn't split sounds too much as it can often mess with the dynamics, so I can't recommend using too many multiband plugins. Instead, try to think about what needs to be wide and what doesnt, keep it simple. In most cases it's enough to just make the whole sound either mono or stereo. And remember, it's not the overall width that makes something sound wide, it's the contrast between mid and side. My rule of thumb is to keep everything that has to sound precise (low basses region and drums, lead vocals) more mono, I leave the stereo space for sounds that require it to work (pads, chords or sometimes background vocals).