Ringing issue aside.. I'd like to wholeheartedly recommend the Schiit Vali for the HD650. I had the Vali once before (right when it was released).. and sold it only because I needed to allocate funds for another HF related purchase. Since then, I've missed its wonderful sound. For kicks, I bought it again last week (the 'ringing' issue on this most recent Vali seems almost completely eliminated) and I absolutely love it with the HD650. Among the many superlatives I can spout off, it was refreshing to witness the Vali's incredible resolving ability, phenomenal tonal accuracy, smooth dynamics, and extremely black background.. the Vali's ability to sound so natural, especially with the HD650 is quite simply addictive. I feel a HQ DAC is important to hearing just what the Vali is capable of, too.. I'm using the Geek Out 1000, atm and I find this combo to be thoroughly satisfying. The Vali is truly a '(hybrid) tube amp done right'.. at under $150, no less!