I found a good 2-channel preamp upgrade to make more difference in my system than when I upgraded my amplifier.

Maybe do some research on a nice preamp in your price range and worry about the amp later. Perhaps a Musical Fidelity preamp, or a Rogue Audio tube preamp. I'd recommend used, as you get much more bang for your buck. I've had great luck on Audiogon buying great gear and prices I was happy with.

I started with a Carver receiver running the Sierra-1's in stereo. Then I used the Carver as a preamp and then picked up an Adcom GFA-545 mk1, which you can find for ~$100-150 and it really sounded nice. A noticeable improvement over the internal amp in the Carver. I was going to build a DIY amplifier and bought an amp to use for its chassis, power supply, and heat sinks, but ended up just using that amp and liked it more (not drastically, but incrementally better everywhere) than the GFA-545. It was a SMART TA-242 stereo movie theatre amp, and it is an upgraded clone (torroidal transformer, etc) of the Hafler amps like the DH-220 and XL-280. The SMART amps are on the bay all the time for ~$200 or less and the Haflers in that range also I think.

After a year or so I stepped up to a sweet 2-channel BAT tubed pre-amp. WOW!!! Talk about an increase in SQ. More improvement than going from some Bowers and Wilkens bookshelves to the Sierra's, or from the Carver to an outboard amp.

Another year later and I upgraded to a BAT tube amp. Another big increase, but I think the preamp was the bigger step up in SQ.

Just depends what you want your system to do. My system is 2.1 only, and I use it for TV/DVD and music. I have a Velodyne SMS-1 for subwoofer control. If you don't absolutely have to have 5.1, you could put the money you'd spend on speakers and wiring into a preamp and/or amp. I'm not worried about having 5.1 at all, and am extremely happy with my setup. So happy that I don't see changing my preamp/amp combo for many, many years to come.

Mike