I don't have a Mainline, so I can't say. Doc has said several times that he finds the Mainline outperforms the Crack with the HD800s. This may be true, but I cannot oversell how well the Crack meshes with the HD600s (and HD800s). People around here are probably tired of me saying it, but I have tried amps costing over $2,500 (with tubes) that the Crack easily outperformed with the HD600s and HD800s. The same is true of amps I've owned that cost $1,700, $1,500, etc. right on down the chain. There is just a special synergy with high impedance Sennheiser and Beyer cans.
I suspect the Bottlehead DAC plus Crack would make you happier than a Mainline by itself.
Regards,
Adam