Does a US amp work in Europe? It depends on the amp. Some are rated for 110-120 volts, so no. Some are rated for 220-240, so yes. Often companies make European voltage or American voltage versions of the amps, the only difference being the transformer. But there are also 110-240 volt headphone amps (I'm pretty sure), which would work fine in either. When you are looking at a specific amp, just be sure to check the voltage.
As for an amp for the HD650, I'm not the best person to ask. Some people thought the CanAmp worked great with these Sennheiser models, but the specific reviews I found just now were from the CanAmp's "honeymoon stage", so it may not be the most accurate picture for how well they would match.
I just know the CanAmp is good for AKG (and other headphones), and the Mapletree amps are good for Grados (and even more other headphones).
Other options out there ... I've heard the Headamp Gilmore Lite is pretty good. But this is by reputation, not actually auditioning it. What I understood was that the CanAmp was better than the Gilmore Lite at roughly the same price range, but the Headamp GS-1 would trounce the CanAmp (yet it costs twice as much).
Edit: One way is to Google "HD650 headphone amp" and find the threads in this forum and others dedicated to this specific question. Try this thread
, for example.
Originally Posted by pataburd
I'm putting in a plug for the Darkvoice 337. Without a doubt, one of the best, if not THE best match I've ever heard with the HD650.
(From a different thread.)