I'm new to the world of good headphones, so I was excited to receive my HD 6xx's and my O2 amp(standard gain)today. I was less than blown away, however, because even at the higher gain, I am listening at(at least)85-90% volume to get good sound. I know more power usually means better sound at lower volume. Can someone recommend an amp that I won't have to max out to drive these things-without breaking the bank? Thanks.