The graham slee solo seems to be quite a good choice for these cans (listening to this combo as I am writing this
). Imported to the US this is probably a bit over your price range but a used one should fit in your budget. I am quite new to this hobby but judging from the research I did on this issue there seems to be something of a consensus that the slee is a good fit with the 650s.
Originally Posted by granodemostasa Meier HAI MKII is also pretty good, the meiers do well with the senns i think.
Jan Meier right now has one as special offer on his homepage, for 240$ (200€) only! If i wouldn't have an Aria already......
But it's one piece only, and it's perhaps already sold. But totaly worth checking it out! (it's under specials, not amplifiers)
I've been usinga Graham Slee Solo with my 650's for quite some time now and have no urge to change it. Sennheiser use it to demo in the UK and it works extremely well although you don't see a great deal about it on these forums.
Beautiful, clear, warm sound with plenty of punch.