I would recommend a pair of Conrad Johnson Premiere 12's used. I believe there are a couple of pairs listed on this site close to your budget. You can get more info at their web site.
Check out the Atma-Sphere MA 60 MK II's. Theres a pair of these beauties in the classified section. These are QUICK, detailed, and with a FIRM, tight, bottom end. Timbre is right on the mark, AND, they are STABLE,STABLE,STABLE. Visit the Atma-Sphere web site, check out the reviews, and most of all, LISTEN to these babies sing. I consider these to represent some of the best amps I have ever listened to. I do not own a set myself, but... someday... maybe even a pair of MA-I's (100 WPC, and very similar sounding to the 60's). Anyway, have fun with your quest for "the sound".