Best integrated with pre/main out/$650

I am with Patriot - the NAD 317 is as good as any amp upto $2000. I think the Happy Medium (happymedium.com) sells 'em for around $600 - right around your budget. BTW it is twice as expensive as the NAD C340 suggested by Doctorjest and if you know NAD, you know that the extra money didn't go into cosmetics. You will get terrific dynamics, articulation and far greater-than-you-should-expect-at-the-price refinement.

Go to the Artech Electronics web site; if they have any left they are clearing out Audiolab 8000A's at $449 and 8000S's (MSRP about $1100) at $549. I use a 12 year old 8000A with a Cardas Golden Power cord (purchased the 8000 used and payed almost as much for the power cord-a good aftermarket cord will do wonders for these amps) and it's a great combination. I think you'd have to pay double anyway to get much better sound.

Best sounding and built in that range is the Audio Refinement Complete. Designed by YBA. You could get a used on for around $700.