All the high impedance Sennheiser cans (HD600, HD650, HD700 and HD800) sound great with the trio Bottlehead amps (Crack, S.E.X and Mainline). I own them all except the HD700 unfortunately, and while I have heard the HD700 previously I didn't hear them on the Bottlehead amps so I can't comment on how good they sound. I never heard a single bad comment with the HD700 pairing with any of the three Bottlehead amps though.
The Crack is really good, but if your wallet allows you to shoot for the S.E.X or the Mainline you should do so. The Mainline is my favorite amp of the three but the S.E.X is pretty close behind. For some reason I personally prefer the HD800 on the S.E.X to the Mainline for symphonies. In my opinion the Crack sound can be described as intimate and fun, while the S.E.X sound is very atmospheric and the Mainline produces the most realistic and honest sound I've heard.
There is a great "Bottlehead amps" review by Loquah on Head-fi that you want to check.