Pros: gorgeous mids, nice extension top and bottom
Cons: doesn't quite have that "liquid" clarity of the best 'stats
This is a review of the Crack amp driving Sennheiser HD800s. I haven't had the chance desire to audition it with any other cans. The price is for the amp ($213) and the Speedball upgrade ($110).
I've had the combo for about two years, and have several hundred hours on it. The Crack is an OTL headphone amp kit, designed for high-impedance phones. Assembly is easy, the directions are excellent, and support on the Bottlehead forum from Dan Schmalle and his very active, knowledgeable customer base is superb. After living with it for a while, I added the Speedball power supply upgrade; it may have made a slight improvement - hard to say without being able to A-B it. I also replaced the stock 6080 tube with a WE 421A, which made a definite improvement - everything opened up more. If you go that route, be patient: these tubes come up pretty often for under $50, and sometimes half that, while there are always a few hugely overpriced examples available for someone in a hurry. Besides, the WE tube looks cool. I know, looks don't matter, but this tube, with its retro Coke-bottle shape and original "lightning" Western Electric logo on the base is just too good to pass up. I forget which 12AU7 I have and the writing is long gone - probably an old Mullard CV4003.
So how good is the combo? It is close to the best electrostatics driven by great tube amps. Mids and highs are superb: flat, extended, smooth, detailed, and never tiring. Bass is very natural sounding, and, thanks to the 800s enormous geometry, never gives that "pounding on your eardrums" effect I get from almost all other cans. Very deep bass is there when called for, with no trace of boominess. Electrostats will give that extra bit of "liquidness" but always at a cost - uncomfortable, hard to drive, you know the list. The Crack on the 800s gets you very close, and with a comfort level not matched by any other phones, at least on my head.
How much of the good stuff is due to the Crack? Quite a bit, I think. Listening through a BGW 100A amp, everything sounded just a little flat. The dynamics didn't bloom as naturally, though, typically for the old BGW stuff, its mids and highs were as smooth as any tube amp. I thought maybe the Crack was smearing things a bit, but that is not the case: recordings I'm very familiar with reveal all sorts of new detail with this combo, but in a way that's addictive. Example: intending to listen to a cut or two from the brilliant Speakers Corner re-pressing of Stevie Wonder's Songs in the Key of Life, I'm now finishing side four. And to those who say the HD800s or the Crack is bass-shy, listen to this record on good gear; you will never say that again. Oh... session files or tapes made with a real EMT 140 plate will give you goosebumps - you can almost "see" the waves traveling in the steel.
Recommendation: if, like me, you spend far too much time with headphones, add a Corda crossfeed circuit to the input side of the Crack - find the schematic for free on the web. There's plenty of room to add a rotary switch with the caps and resistors hung directly off it, but that gives you a rat's nest, so be neater than me and put everything on a separate board. The Corda circuit lets you dial back the separation, which makes the presentation of some material much more natural.
Associated gear: modded Thorens TD125-II, modded Hadcock 242 unipivot arm, MusicMaker m/m cartridge, modded VTL Deluxe preamp, Oppo BDP-95, modded Otari MX5050BII, Protools 24/96 sessions through a Metric Halo ULN2. Electrostatic headphones include several Stax models from the 1980s, and the original Koss ESP-9s: all memories now, but vivid. Monitors are modded Spica TC-50s driven by Manley 120s, with a modded ACI powered subwoofer and custom crossover. Occasional monitors are teensy Braun/ADS L300s, which still amaze after 35 years, driven by a modded BGW 100A amp. All interconnects, speaker, and power cables are suitably pretentious/expensive stuff that might be better than zip cord, or not.