With another day of listening under my belt, I figured I would offer up some further Mainline impressions - which I have also added into my post (#7) on the first page of the thread. I've also really been enjoying reading all of your respective comments on the Sex + C4S and greatly look forward to Loquah's Mainline vs. Sex head to head once he's got them both sitting side by side.
To my ears, Mainline retains much of what I have come to think of as bottlehead's house signature (fast, dynamic and engaging - making your toes tap - with a hint of tube fullness and warmth - the tubes, in a sense, filling in for the room effects you don't get with headphones). However, in the wake of several years of daily listening to my Crack, Mainline is just incredibly clean and refined in comparison, particularly in its excellent bass response and handling of transients. This is no doubt the result of its ultra highly regulated single gain stage topology and excellent custom output transformers. Based on my recollections from CanJam - I would analogize that mainline is as fast, detailed and dynamic as a Zana Deux (leaving slower "tubier" flagships like the Woo 5 in the dust), but with a slightly sweeter top end. Trumpets can still scream when the source material calls for it, but I can't imagine this amp will ever be characterized as even slightly harsh or piercing - which criticisms are occasionally aimed at the Zana. (I imagine this may mean that mainline has nice low high order distortion - sort of the antithesis of the disproportionate glare folks complain of in SS amps with lots of global feedback?) In any event, these positive qualities essentially mean that the mainline is a perfect match for hd800's to my ears. Even when listening to very simple music (i.e. the Wailin Jenny's - the Parting Glass), the incredibly clean separation between voices is an obvious improvement over other amps with which I am familiar and makes vocal harmonies that much more enjoyable. The same obviously applies to fast complex music as well, where the mainline's control and handling of transients is consistently impressive. The more I listen, the more enamored I become.
Hope more of you will have a chance to hear this amp in the near future. If possible, I will bring mine to the So. Cal. meet in March. Also looking forward to receiving the balanced Alpha Dogs I have on order and reporting back as to how they pair with it.