@grm -- I bought a Quicksilver Audio headphone amp a few months ago. I have the Senn HDXX (same as 650 and somewhat similar to your 600), Focal Clear and Hifiman Arya. The QS amp was a night and day difference. If you read my other threads, bass is one area where I saw the most noticeable improvement. It is an amazing amp!!!
As for your 600s, all I can say is that although my other two headphones are much more expensive than my HDXX, the Senn punches way above its price point, and definitely scales with better amplification. Try it with the Quicksilver amp, I think you will be pleasantly surprised what the 600s are capable of.
As for your 600s, all I can say is that although my other two headphones are much more expensive than my HDXX, the Senn punches way above its price point, and definitely scales with better amplification. Try it with the Quicksilver amp, I think you will be pleasantly surprised what the 600s are capable of.