I'd recommend trying new main speakers. A good efficent speaker would be a better upgrade then adding more power to a so-so speaker. Try Paradigm speakers as a cost effective choice. Klipsch are pretty good, or Definitive Technology.
If you do go with mutli channel amp (under $1200), check out ATI, Outlaw, Bryston (good amps, but expensive compared to others I mentioned) or yes, B&K.
If you do go with mutli channel amp (under $1200), check out ATI, Outlaw, Bryston (good amps, but expensive compared to others I mentioned) or yes, B&K.