Just a heads up: starting from May the 5th thru the 11th, the Bottlehead Crack amplifier w/ Speedball upgrade will be on sale, priced at $299 rather than the usual $414 on the Bottlehead online store. This is quite a rare occurrence, so anyone with 6XXs (or waiting for a pair like myself) should seriously take advantage and get one.
FYI, the Crack w/ the Speedball upgrade is regarded as the very best headphone amp below $1000 you can pair to the Sennheiser HD650, and by extension to the HD6XX.
So if you can, get one now. I will certainly do so.
nsonnsonCrack and Audio-GD has quite different sound so it's not really fair to say which is better.
Usually for Sennheiser dynamic you should go with tube, unless you don't like tube.