Thinking about your question another way the Cracks ability in transparency is beyond that which my HD650 in standard form can resolve if that's any help. Plugging the T1's into the Crack and it will resolve details that my own HD 650 clearly misses and the Crack 650 combo is well know for its superb synergy they really are a killer combo..
Thanks for the reply!
O2 is quite quite popular solid state amp (not tube). Its synergy with HD650 is also quite popular. So, wanted to check if anyone who has used both (O2 and Crack) can provide some input. My current rig is DX100 > O2 > HD650.