I too have been a 650 fan for years and have just upgraded to the He500's. I wouldn't read too much into them being really hard to drive, they play well with an amp that gives about 1 watt into 32ohms and I think (don't quote me) that the Burson does just this. I'd say they are easier to drive than the 650's, I didn't think this to begin with as they sounded nice but dull for the first few days but after time they really opened up (at least 100 hrs). I'd also say that solid state is probably the best option for them too. They have a nice laid-back character and I think SS compliments this better.
If you love the 650's then your really gonna love the HE500's. They do everything the senn's do, only better. It's the cohesiveness that the 650's do that has always amazed me, they make the music whole and enjoyable. The He500's do this too, but with more realism, detail and refinement. More light and shade. Very balanced (don't want to use the word "neutral" stupid word in audio), nothing stands out or seems lacking. My only hyper-critical issue that I mentioned in another thread is that the lowest FR could be tighter. In a perfect world they would be, but we are not in a perfect world. So in that sense I state "the HE500's are perfect"