Again, everybody has their own idea of what magic sounds like but I don't have any ss amplification in either my main speaker rig or my headphone rig. To me, the Crack in its stock form (and Doc sends out some quality NOS tubes with his kits although you can certainly tweak the sound to your liking with aftermarket stuff) is miles beyond anything SS.
Lots of people put lots of names on the sound but tubes are a warmer, more "natural"-sounding approach so if you're looking for the last word in sonic precision and accuracy, tubes probably aren't for you. But if you want something that allows you to just listen to the music with a lot of enjoyment, tubes have no substitute.
As with anything else in this hobby, it's all in the ear of the beholder. The best thing to do is just try things out by whatever means you have available - I had tube speaker amps way before I had anything tubed feeding my headphones so I had a head start - and figure out what you like. But if the reviews of the Crack and Senn pairings are to be believed (there are many and they're all pretty right-on if you ask me), it's like the two should be bundled together. The sound really is that good.
Have you finished your Crack build, stephenbou? The day I finished mine and plugged in that first set of high-impedance cans is probably in the top 3 audio milestones in my life.