I had the 500s in the house again last week. While they certainly are top notch headphones, I missed my 6s. They are back on the job and I bid fare well to the 500s. I think I might see them again as a replacement for my RS-1s, but not before upgrading my DAC.
Transatlantic "Shine On You Crazy Diamond" is putting a grin on this happy camper.
The HE6 is better and especially if you like solid state. But for tube amps the hE500 is the ticket because they are easy to drive. The biggest differences are more transparency deeper bass and more extended treble but you need the juice due and with the 500 you can get nice results with one watt. One watt wont cut the HE6 you need 8-15W to really make them shine IMO.