Interaction between Sub XO and Amp?

I want to use my sub's crossover to cut off the lowest frequencies from my main (bookshelf) speakers. The concern I have is that the sub's amp will process the frequencies above the filter, resulting in a degredation of the sound quality.

Optimally, I'd like to use the line-level in/outs to run preamp out1 to the sub, then the line-level outs on the sub to the amp (and amp to the main speakers.) If the crossover works "before" the amp, and simply passes the frequencies above the cut-off to the amp, life is good. But if the amp gets involved and "corrupts" the clean, accurate signal my preamp is sending I need to decide whether to buy a sub whose amp won't degrade the signal (much) or just go with floorstanders despite a fairly small room (12x17).

Thanks for your help!

Howard