Hi all, it's done!! I've finalised the Mainilne review and comparison of all 3 Bottlehead amplifiers: http://www.head-fi.org/t/683012/bottlehead-amplifier-discussion-comparison-thread-crack-sex-mainline#post_9831533
I thought an overall summary of the 3 amps might be helpful too so here goes:
Crack is easily worth more than its price tag and offers outstanding stock performance with the opportunity to scale up using various premium tubes. The Speedball is a must-have in order to hear the Crack at its peak and it's a cheap, easy and reliable upgrade. Easily the easiest Bottlehead kit to build, even for beginners, the Crack is a highly recommended addition for anyone with a pair of 300+ ohm headphones.
More versatile, more powerful and more spacious - the S.E.X. is great if you want to use speakers and headphones from a single amplifier. It can run lower impedance headphones while still having plenty of power for high impedance and / or low efficiency cans. It's definitely a harder build than the Crack, but just requires time and patience, not greater skills. Similar to the Speedball for the Crack, the C4S upgrade is a must-have for the S.E.X. and transforms the amp from good to outstanding in a matter of minutes (it's a simple kit you can build and install in <1 hour). I'd recommend to the S.E.X. to anyone looking to spend less than $1000 on a headphone amplifier to drive a range of headphones.
The easily justifiable flagship of the Bottlehead range is harder to build than the Crack, but not vastly different than the S.E.X. It's sound is much more akin to the Crack than the S.E.X. and is intimate rather than spacious, but it's presentation is so perfect that imaging is outstanding despite having slightly less stage space to work with. The Mainline will easily fill the role of end-game amplifier for many people, especially with its ability to drive balanced or unbalanced headphones and with switching to optimise output impedance to suit the headphones used. As always with Bottlehead products, the kit design means you get the satisfaction of building (and possibly customising) your own amp as well as an incredible price-to-performance ratio.