dunno, I listened to the beta22 and my PPAv2 with the HD-650 and really think that it doesn't do them justice. Especially the highs, they're painfully bright and piercing, even for me. I've heard them on the Headamp amps as well and was equally unimpressed. Now, the 600s I think do better on solid state than the 650s, but I have to say I may be in the minority here then