If you look over on headfi you will see those cans have met with a response that places it in distinctly lesser offerings. There are a few threads on the SQ there, fwiw. I've learned to take all such input with more than a grain of salt, but certainly have found one can get a pretty accurate overall impression as feedback builds up. The Consonance amp I just sold (Cyber 20) is a damn good little amp for the money. I'm not sure I know what you mean by an "aftermarket tube hp amp". There are some nice tube hp amps coming out of Taiwan (DarkVoice), no experience there but plenty of 'followers'. I can tell you first and that Woo Audio stuff (made in Brooklyn) is top notch. One thing about the good HP amps, they seem to hold their value on the used market and are actually in demand. Check out the internals on this Woo...like I said, very nice stuff. I'm using a maxxed out Woo WA6SE and it pairs wonderfully with AT W5000 cans. The dual-volume request you had is going to be a difficult one to find. I haven't even seen HP amps with a balance knob on them. In fact, the trend is to include an optional filter switch that allows sending some of the right signal left and left to right to blend the channels to some extent. This is to try to compensate for the psycho-acoustic effects that happen when a very exagerated spacial recordings have when listened to on HP...something like the first cut on Sgt. Pepper, or much of Pink Floyd...those kind of spacial exagerations don't play well with your brain on headphones, and it sounds better to blend the two channels to some extent in those cases. Not all HP amps have that option. Meier-audio would be one that does. A mono option would be a nice feature too. Not many amps with that. I personally HATE dual-volume controls on preamps, and I have no doubt I'd hate'em on a head amp. They require too much attention to fiddle with every time you want to make a volume change. Two source inputs are fairly common. There are also some good headphone amps that double as conventional preamps (see Woo 2 and HeadAmp's GS-1 and GS-X, among others). Also, of course, some full on amps with great headphone stages (Woo 5, Portal Panache, Cary various, etc.).
Here's some info from the Meier Audio site on crossfeed filters. I can't provide a direct link and it was a bit difficult to ferret out, so forgive the bandwidth and skip this part if HP listening isn't of interest:
Here's some info from the Meier Audio site on crossfeed filters. I can't provide a direct link and it was a bit difficult to ferret out, so forgive the bandwidth and skip this part if HP listening isn't of interest:
In normal daily life people use various mechanisms to locate sources of sound.
Firstly, the sound of a source to the right side of the listener (e.g. the right loudspeaker) not only reaches the right ear but, attenuated and delayed, is also heard by the left ear. The level of attenuation and the delay time of this crossfeed signal provide important directional information.
Secondly, the soundwaves are partly absorbed and partly reflected by the tissues of the head. Reflections at the oracles (pinnae) interfere with the soundwaves that directly enter the ear-channel and amplify or attenuate specific frequency components. Since these reflections depend on the direction of the soundwave the "color" of the sound changes with the direction of the source.
Thirdly, reflections of the soundwaves from the walls, ceiling and floor of our listening room produce reverberation that conveys an extra feeling of space.
The information obtained by these mechanisms is further refined by movements of the head. Changes in sound levels, delay times and sound color refine the sense of direction. For a demonstration, blindfold a friend and ask him to locate a ticking clock that you have hidden in the room. He will start turning his head although he can't see anything. With his head in a fixed position an exact localization is much more difficult.
Listening by headphones
All the mechanisms of directional listening are missing when we use headphones. The sound at the right ear will no longer reach the left ear and pinnae-reflections no longer interfere with the original soundwave. Moreover, the headphones are directly attached to our head, and so head movements no longer add information. Reverberation is also not present.
As a result, the sound heard by headphones seems to stick to the inside of our head and to our ears and an unnatural soundfield is created. The brain misses logical clues for direction and this subconsciously results in mental stress. Some people cannot tolerate this stress and are unable to use headphones.
The natural crossfeed filter
In principle, digital soundprocessors can simulate all the mechanisms for directional listening but the results are, thus far, not very satisfactory. In particular, pinnae-reflections are very complex and listener-specific and impossible to simulate accurately.
Fortunately, the mean directional information is provided by the time delay and level of attenuation of the sounds that reaches the opposite ear. The CORDA headphone amplifiers can electronically simulate this process and, with appropriate attenuation and delay, add some of the right audiosignal to the left channel and vice -versa. This considerably reduces the adverse symptoms of headphone listening.
A unique feature of the crossfeed circuitry of the CORDA headphone amplifiers is that it "recognizes" the virtual positions of the instruments and singers in a recording. The sound of an instrument in the middle of the soundstage will be equally present in both audio-channels and isn't given any crossfeed. A crossfeed signal is only generated for instruments that are not placed at the center. The more off-center the instrument is placed, the stronger the crossfeed and the longer its delay. This feature is called "natural crossfeed".