02-20-09: MapmanI had a Musical Fidelity integrated, and have heard several of their integrateds at the local high end store, and I like the Onkyo better. Personally I hear a little bit of midrange glare in MF amps and most Cambridge components as well, which is completely absent in the Onkyo, while the Onkyo matches or beats them in low noise floor, transparency, and low level detail.
I like the Onkyo suggestion.
Another good choice in that range might be a second hand Musical Fidelity Integrated.
Or how about a tube hybrid like the Unison Unico, or something tubed and digital-ready with a nice built in DAC even like the PeachTree Nova?
Mapman
I heard a PeachTree recently and was very impressed, both for its linestage and its built-in DAC. I think the PeachTree is a great choice for a small digital-based system. Teamed with a Wadia iTransport, it can make an iPod sound breathtaking when playing lossless files.
The OP, however, was looking for a good amp for a vinyl-based system. For under $800, you can't do better than the Onkyo, Another alternative would be the Denon PMA 2000IVR. Some owners swear it has one of the best phono stages they've ever heard, certainly in an affordable integrated amp. The Denon is certainly a different beast, weighing over 50 lbs. and needing about 4" of cooling clearance on top and sides. It also has a big footprint and won't fit on every rack out there. Still, I bet it sounds sumptuous.
Even if you got a Cambridge, you'd have to get an outboard phono stage, and when it comes to that, the one built into the Onkyo is better. I find Cambridge integrateds to have a bit of glare like the 640P does.