Sound quality impact - preamp vs power amp?

Preamp gets the nod, as regardless of the amp the sound quality can be no better than what the preamp feeds it.

Yes, a high sound quality amp can sound better with the same preamp, however, that is a result of the the lower sound quality amp NOT achieving the sound quality feed to it from the preamp.  

Cost wise, a match of preamp & amp sound quality will result in the best sound quality for the BUCK.