options for 2.1 sound

I have a 2.1 channel system with bookshelf speakers, a pre amplifier and a 2 channel power amplifier and a subwoofer. Currently I have an adcom gtp-600 surround pre amp but I'm not satisfied with it (I'm actually quite confident that it's damaged in some way.) I want to get a new pre amp and am wondering what the best solution is. I know that I can get a normal 2 channel unit and run the speaker cables through the sub but I'm hesitant about this because I don’t want to tarnish the power going to the speakers by sending it through the sub. I could opt for another 5.1 channel unit (outlaw audio 950?) but then I'm paying for capacity that I won't use, and likely getting lower quality 2 channel sound than I would with a stereo pre amp. I also have been told that I might be able to employ a 2 channel pre amp that has multiple outputs (Adcom GFP-565?) and send one to the sub and one to the power amp. I’m thinking this may be the best option. Any advice/ help would be greatly appreciated.

Bill.