I would really appreciate some advice as to how to improve a music system that came with my house.

Current setup.
5 rooms (3 bedrooms, 1 office, 1 bathroom), each with one speaker in the ceiling (so I'm guessing mono). Each speaker has a volume control knob on a nearby wall and a wire goes from each speaker to a central location where an ancient looking tuner/amp lives. 2 speaker wires go into the SPEAKER 1 LEFT +/- holes on the back of the amp, and the other 3 into SPEAKER 1 RIGHT +/-. So all speakers play the same thing, whatever the amps source is set to. And I suspect jamming all those wires into the holes is not recommended.

Desired setup.
I'd like to add a 6th speaker in another room (mono or stereo). I'd like to listen to the mp3's on my server, or on a USB stick. I dont really need any other sources. I dont mind if all rooms play the same source, because I really only want to listen to mp3's when in these rooms. The amp location is right next to my patch panel so network access is a short patch cable away. I have limited vertical space so cant go piling component upon component. 2 units max, 1 would be better. I dont want to spend a fortune.

What I have found out so far.
I can get a 6 zone speaker selector for about $100 (Rapollo 6 channel speaker selector). I'm probably going to need this regardless.
I can get a network audio player for $1000 but it needs an amp (Cambridge Sonata NP30).
I can get a network A/V receiver for $800 (Onkyo TX8050), but I dont need all those other sources.
I can get a network audio player with built in amp but they cost thousands (most of the things on this page!)which is bizarre considering 2 separate components costs sub $2000.

I feel like I've googled myself into a corner, so if someone out there knows a better way I'd love to hear about it.