You want a 2-ch power amp for <$300. You also need 2-ch. preamp. How much are you willing to pay for the pre--$200? That's a total of $500. You can get a new integrated amp for around $500 that is better than pretty much any used amp + preamp at that price unless you get extraordinarily lucky.
I have been integrating systems on a budget for 14 years. I have stacks of old surround processors, preamps, and power amps. Some work, some don't. I paid $200-400 each for the better power amps, and $200-ish each for a couple of preamps. These units were state-of-the-art when they hit the market.
They're all on the shelf now because I have a new Onkyo A-9555 integrated amp that I got from an authorized dealer w/warranty for $474. The Onkyo is faster, more transparent, organic sounding, more stable, clearer, and has a much lower noise floor. It can also deliver an incredible amount of instantaneous current, making the amp sound more powerful than it is. It's about 100wpc into 8 ohms and 200 into 4. You cannot bi-amp with it because it does not have preamp-outs, but you can easily biwire with it, and that's what I do with excellent results. In my experience, passive biamping and biwiring offer pretty much the same sonic benefit over single-wiring, minus the additional 3dB headroom you *may* get from bi-amping. (Another candidate would be the Cambridge Audio 640A v2 or 740A in the same price range.)
As I said, you'd have to stumble onto a rare deal for less to get better sound from the used market than what this amp offers new. And face it, Carver, Adcom, and Parasound are well-known and nobody gives'em away.
