I also have the Crack (with 1/2 Speedball upgrade, 5998 + Brimar 12AU7) and Mainline, and I have both the Senn HD600 and HD650. I agree with you about the Crack + HD600 combo. It really is wonderful. For the Mainline, the added mid-bass of the HD650 pairs well, as I find the Mainline bass a bit on the light side for my taste. I've read on these forums that Mundorf Supreme caps can add warmth, so I'm thinking of replacing the stock Mainline output caps with those.
-Raymond