Someone should also mention to watch for phase issues when placing your microphone(s) using two or more amplifiers at once (or when re-amping).... It is even possible for two amps to have opposite polarity (180 degrees out of phase) due to different internal pathways... make sure you try hitting the "phase button(s) on one of your pre amps... (see if it sounds fatter, if it does then maybe keep it in)
I agree on the no "Y" cable... possibly a good way to get ground loops and serious hum I think. Some isolation like a transformer needed....
I would also say that I seem to find using multi amps, or re-amping, to pose different problems each time... hums , phase , etc. ... but with patience, and trial and error, it's worth it!