There is no question that Paul and Doc know the answer to this better than just about anyone, but I have to argue for not getting rid of the Crack too quickly. I am about to build both a Smack (discontinued) and a S.E.X. 2.1., but I don't see me getting rid of my Crack any time soon. With high impedance headphones, and Sennheisers in particular, I consider the Crack a small wonder. I have yet to hear a better amp when paired with my HD800s and HD600s. As for the HD650s...I have owned (and sold) three pairs. Each time, I sold them because I found the HD600s to be more transparent. I find the HD650s to have treble roll-off, or at least muffled, inarticulate highs. Yet this is one of the areas where the Crack excels. If any amp can wring the best from the HD650s, I suspect it is the Crack. The Mainline and the S.E.X. may be better amps overall (I will reserve judgment on the S.E.X. until I have built mine and can do a first-hand comparison), but I find it hard to imagine that either will complement the HD650s as well as the Crack.
I suggest you build the S.E.X. before making a final decision on your Crack.
Best regards,
Adam