I can remember working on a very early Crack prototype a long, long time ago for a musician who wanted a high end headphone amp. I remember sitting down and listening to at least 15 different tube operating points, then I finally found one that just sounded really good. I took voltage measurements, scratched my head a little bit, then realized the operating point was one that I had run into building Bottlehead kits with the same tube.
The biggest improvement to be had in the Crack, beyond the Speedball, is shunt regulation. Unfortunately, you really need a bigger power transformer and chassis to accomplish that.
-PB