Receiver for in-wall speakers, any difference?

I'm helping a relative who is trying to get better sound from a system that was installed when they purchased their house. The system consists of a cheap sharp receiver with JBL in-wall speakers. The speakers sounded pretty nice playing cd's except the receiver sounded weak. The system is in a 700 sqft. living room with 20ft. ceilings.
My question is: Would a better 2-channel receiver with more clean power make a difference?? Should I get a receiver with a sub-out?? I've never used in-wall speakers and don't know their limitations or how much power you can feed them? Any suggestions would be appreciated..