The problem with mono output synths is exactly that - they sound mono. In the past I've tried to artificially create a stereo effect using the mono recording but it has never sounded very good, until I stumbled upon this neat and simple trick.
Simply record your audio in twice. Then pan one hard left and the other hard right. This will give you a full width stereo recording and it will sound lovely.
CONTROL THE WIDTH
You can then apply a plugin such as Utility (Ableton) or S1 Imager (Waves) to reduce the stereo width if you desire.
This is particularly effective on basslines. You could record a Minimoog or DX7 in using this technique. In this instance panning hard left and right will create space in the mix for the Kick drum.
I personally use this technique on all the recordings I make from mono output synths. The results are perfect.