Really depends upon your budget, but you can start with less expensive units and see how it goes from there.
Within the Marantz line, you can try a 2010 or 2015 - lower wattage, so many using speakers may want the higher powered units, but these should be fine for your HD650's. If you don't want or need radio, you can also consider an integrated like a 1060. All should be attainable for $100-200 if you shop around, and the 1060 is pretty easy to recap/restore.
Outside of Marantz, you can check out some of the Sherwood's (7100, 7200) or Sony's (I've had a 6055, getting a 6060) - they sound pretty sweet and are generally fairly under the radar. These should all be in the same price range as above.
Others can suggest other models, but I'd start relatively slow to start and not spend a ton of $ until you get a sense of what you like and whether you are up for doing tuning/maintenance, and possibly restoration on your own. If you buy an original piece, it may work fine for awhile but generally will require some work in time to keep it going.