Sennheiser used to demo the hd650 with the Graham Slee Solo. I had one and liked it, but nowadays, I'm not so sure that it's great value for money. I saw some performance figures way back for the Solo compared with something else, and it put me off the Solo so I sold it. Mine actually went to China!! (As opposed to the other way around
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I have both an Ember and Polaris and if anything, I slightly prefer the Polaris with it. Power managent seems easier on the Senn and the sound is slightly less 'fruity' than the Ember. On the face of it, many would prefer the Ember and then continue on to go through a barrage of tubes which to me always suggests that they want improvements. The Ember is impressive alongside the Polaris which doesn't sound quite so 'flashy'.
It's that ordinary kind of sound that I actually like. It sounds a fraction cleaner and has less of a tubey flavour, although it is voiced to emulate a tube sound but has less of it. So it's not as 'sqeaky' as some ss amps and has a roundness about its sound that isn't quite as much as the Ember.
I feel that the hd600 is better with the Ember and the 650 with the Polaris.
That's just my preference though, but thought I'd try and let you know why to help you with choices.