From a design standpoint - as relatively efficient, high impedance cans, the HD650's pair beautifully with OTL tube amps - a topology that generally delivers ample voltage swing but not a lot of current. Inefficient low impedance headphones, by contrast, need lots of current - which is easy to get out of solid state and/or hybrid designs. This is why you don't want to try and drive planars, or even K701's for that matter, out of *most* OTL tube amps.
Based on the time I've spent lurking on head-fi over the past several years, my sense has been that the majority seem to prefer sennheisers via tubes, but I've never counted. Certainly, there are plenty of satisfied listeners on both sides of the aisle. A well designed amp from either camp should get the job done, but if you have a chance to attend a meet and do some listening for yourself, I'd strongly encourage you to do so.
Hughkk: with a budget of $350 - I'll definitely join twinster and captouch in recommending the crack. I've been to CanJam and listened to a huge number of amps, and it is definitely the best value with the HD650's and 800's that I'm aware of. If you have a chance, take a look at the comments/excerpts on the BH site from numerous head-fiers, as well as highly respected pros like Tyll and Steve Gutenberg:
http://www.bottlehead.com/store.php?crn=224&rn=442&action=show_detail