I personally love the 6XX/650 but I can't speak for these amps. If you can, try before you buy. I wouldn't even see the 6XX/650 as a startingpoint, because, depending on your needs, it might as well be endgame already. Having heard the HD800 I still have no desire to move on. I plan on getting something from ZMF one day, but the 6XX will be at my side until it stops working and once that happens I will buy a 650 or 6XX again.

I would say go for it if you like non fatiguing, laid back sound. 500ish € would be about the same price I had to pay to import my 6XX and purchase the FiiO K5 pro. Probably on par with what is listed for you, so definitely more than worth it.

If the pads of the 650's are flat, I can definitely recommend that you replace them with either stock pads or ZMF suede pads.