Supplementing my impressions above, here are the further comments I posted in the crack, sex, mainline comparison thread:
The mainline retains 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), but it is just incredibly clean and refined, particularly in its excellent bass response and handling of transients, compared to my modded crack. 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 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 really 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.
And this was Doc's gracious and highly informative response, as mentioned above:
Thanks so much for your kind words, skeptic! I would agree with what you are describing of the sonics. Mainline was a joint design effort of PJ, PB and myself. In some of our designs I let the guys pretty much dictate the way the amp will end up, for example PB's Crack circuit, or PJ's Quickie. However I kinda fussed and poked and prodded at this one to get exactly what I had in mind. I wanted an amp that was capable as a tool in a critical listening environment, that is to say something I could use for evaluations on my job whether they be equipment or recording quality evaluations.
The OTL approach is very attractive because of its potential for great bass peformance. However OTL headphone amps tend to use cathode followers (including Crack). The CF is a great circuit, you get rid of the potential issues you have with transformers (expense being a big one) but I hear a certain quality to it that is identifiable as cathode follower sound. Same story for multiple gain stages like S.E.X. has, transformer outputs, etc. This also applies to DHTs vs. IDHTs and a lot of other things. Any design is a compromise, and what you are really doing in any design is deciding which compromises are acceptable to your personal taste. Hopefully one can find someone else out there who has the same taste...
In this case I felt that we could overcome the potential shortcomings of a single gain stage and having to use an output transformer more effectively than dealing with the sonic artifacts of cathode followers or multiple gain stages. This is the kind of thing you only pick up with experience, you gotta build 'em all and critically listen. I also knew from experience that - though I am still not sure why - the attenuator has a huge influence and needed to be the best we could find. We couldn't find what we wanted so we designed our own, as we did the transformers. Because of that kind of approach to the design Mainline will never be inexpensive like Crack or versatile enough to drive speakers like S.E.X., but I do think it is our best sounding headphone amp.
In any event, I aspire to continue updating this thread and cross-posting relevant info to the extent it relates specifically to the mainline (in the event that future readers and potential builders lack the time to read through the entirety of the bottlehead comparison thread). I continue to soak up as many listening hours with mine as I can squeeze in on any given evening. It is simply outstanding with hd800's.