With Totalmix FX, if you don't have Submix mode engaged and you don't have a "direct down" routing flow, you can end up hearing both the original unprocessed signal AND the return feed simultaneously, which will naturally sound very phasey and louder (watchout for evil endless feedback loops from the input faders being up).
Totalmix is really powerful, but if you're trying to do any send/return kind of routing and you don't understand its workflow (me a couple of weeks back!) it can create nightmares!