The following post if my opinion. Based on my experience with these amps.
I've used Crack, S3X and Mainline with the same DAC/pre-amp the last few months on my desk. Feeding Sennheiser HD800's (SDR) most of the time. Second choice are AKG K340's.
What i do note is that when i use more "hi-fi" tubes with Crack, like the E80CC or 5692 Ayadula mentioned above (also listening to that right now!), it's not that much different from S3X or Mainline. What i mean to say is; i'm not missing anything with Crack. It's all in there, just a different presentation.
Ranked in order of use:
1. Crack (did anyone mention that tube-rolling is fun?)
2. Mainline
3. S3X
Even with Mainline and S3X next to it, my Crack gets most usage.
Ranked in order of sound quality:
1. Mainline
2. S3X
3. Crack
This is a no-brainer, if you are looking for the best. Get a Mainline. It is the best. Period.
Ranked in order of versatility:
1. S3X (it's moving again to a different room, for speaker use only).
2. Mainline
3. Crack
S3X can feed speakers aswel as headphones in all genres. I've listened to it with various efficient speakers, B&W P9's, HD800's and LCD3's and all sound good with S3X.
Ranked in order of learning experience:
1. Crack
2. S3X
3. Mainline
Crack is the most simple circuit, with the most room for improvement. This makes for a lot of opportunity to learn about circuitry and the effect of certain parts on the actual sound.
As the volume-pot mentioned above. The Bottlehead team put in a good basic volume pot and left enough room in Crack to let the builder put in any pot they wish for.
Ranked in order of sheer fun:
1. Crack
2. Crack
3. Crack
For me, the fun in Crack is derived for the majority from the learning experience. The other part is tube-rolling. Crack is made for tube-rolling.
Ranked for ease of build.
1. You can build them. All three.
2. Yes, you read correct. The manual is so good that with determination everyone can build a Bottlehead kit.
3. I build my Crack with 0 experience in electronics (even had to buy my first soldering iron before i could start).
The quality of the manual and support are the most important things about a DIY kit. And Bottlehead in my experience, is brilliant with both. They deliver a very thorough manual, that takes you trough the whole build, step by step. Everything is in there, from the first soldering lesson up to safety instructions on how to check your amp and where to start when you need to troubleshoot.
If you can't figure it out on your own, there's the support forum where the team will stay with you until your problems are all resolved (when needed).
When you buy a Bottlehead kit, your not on your own to build just a kit. You also buy a very complete manual that will guide you and a team that will give advice until you've completed your journey and are 100% satisfied with your build.
Sorry, what was the question? xD