The multi-effects units that do amp modeling can plug straight into the poweramp; they usually have more than enough output to do that. The Line 6 PODs are a good example of that. But even then, they don't tend to like the output of the preamp... so in those circumstances, it'd be best to leave the preamp disconnected, and plug straight into the unit - basically, bypassing the preamp entirely and using the unit as your preamp instead.
In many cases, you can get away with running preamps into each other and not lose output or have distortion issues, but the layers of coloration can be problematic. IMO, of course.
I love my GK amp; I don't really need another preamp. Just more pedals. Mmm... distortion... filters... delay...