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For those wondering why a fully balanced preamplifier would sport a stereo volume control, Xuanqian had this: "You didn't see wrong, it is a 2-channel control (and the reason to have 3 SE inputs was simply because most preamplifiers use such a layout and we had insufficient space to add additional XLR inputs on the back). But it is always better to use an XLR input even if you feed it single-ended signal with pin 3 shorted to ground. I don't think one should single out balanced preamps like ours—or Gryphon's Athena you recently reviewed—for their single-ended volume controls. Even the FM Acoustics FM 266MkII which bills itself as 'the only truly balanced' preamplifier sports a 2ch Alps RK-27 pot.


"One must distinguish between balanced signal transmission and balanced amplification. We're talking about the latter. Then it doesn't mean four individual amplifiers which would never cancel common-mode input interference. A balanced amplifier is qualified by using a balanced input with common-mode cancellation and a balanced output stage regardless of what occurs in-between. To explain this more vividly, let's choose from two amplifier structures:
1. balanced input stage converted to single-ended with common mode interference cancellation -> single-ended volume control -> conversion to balanced signal -> balanced gain stage -> balanced output stage
2. two individual input stages handling hot and cold signal without common mode cancellation -> balanced volume control with attenuation error of 5% ->  two individual gain stages for hot and cold with 5% mismatch ->  two individual output stages for hot and cold with 5% mismatch.
  

$23.500 Esoteric C-02 with quad volume control


"With option 1 and a precision-matched circuit (very easy to achieve*), you will cancel out almost all input interference and achieve very precise gain control on both hot and cold signals. With option 2, volume control gain is mismatched, hence hot and cold signals at the output are unequal and common-mode interfere becomes part of the signal. Worse, this error can't be cancelled out completely afterwards. The greatest issue is that you will never find a perfectly matched volume control whose channels are perfectly gain-matched especially at low volumes. The specification of the expensive DACT stepper attenuator for example still shows 1% error. Then it only has 24 steps and no motor for possible remote control. Most motor-controlled versions aren't stepper types. For a quality unit like the Alps RK27 this means inter-channel tolerance of 5%. Most others will deviate by 10% . Should you try to use such a type inside a structure of four individual amplifiers, you'll have a true nightmare. Most factory-published measurements set gain to max to disguise these common issues but during actual use you can only image what happens. The only way to avoid all this is to use a relay-array volume control or very very expensive precision-matched attenuator. And this will force you to sell your preamplifier with more zeroes before the comma which is another story altogether."
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* Some might insist that achieving perfectly balanced signal halves is rather more difficult.
One chapter in that other story would be the €15.000 counter-parallel floating-bridge Thorens TEP 3800 preamp with its 4-channel volume control which makes for a truly balanced input-to-output signal path on XLR. On the $749 side of the same equation sits the equally cross-shunt/push-pull (circlotronic) N-channel Mosfet Schiit Mjolnir headphone amp with XLR pre-outs and quad Alps pot [left]. These two examples aren't mentioned to contradict Xuanqian. They simply illustrate design decisions which other makers find just as valid or superior. Whether balanced signal transmission in home hifi with its usually short cable lengths is even necessary—studios by comparison run literally miles of cables—is another topic where opinions diverge once more.


Diverging opinion also applies to headphone use. The different treble voicing Xuanqian explained is clearly audible and in effect. And it's more specific than the lit-up-all-over Mosfet-based Bakoon AMP-11R as my current headphone amplifier king. If for example you have original Audez'e LCD-2 and always wished for a bit more cayenne-pepper separation plus fluffier airiness with less chocolaty density but weren't ready to risk twice the coin on the purportedly revoiced LCD-3, the Taurus Pre could swing the deal without a can swap. This greater incisiveness—'3rd-order' mode, pentode flavor or bipolar vs. Mosfet—is aided and abetted by very high detail magnification down to the bone. That must be a function of a really highly polished noise floor. Think recording console in hear-everything active monitor nearfield mode. It cuts out reflections and their resonant blurriness.


That's already a given for any headphone so push this equation a bit further. It follows that inherently brighter headphones like Sennheiser HD800 with stock leash could get too forward though a valid counter could be a deliberately fleshy source like Burson's older DAC. Midrange-centric headphones like my former AudioTechnica W-1000 and W-5000 would get a bit of a light injection. Regardless how you feel about this tonal center of gravity, transferable from taste to taste will be a transient-sharp highly percussive feel that places a high value on drive, tautness and articulation - spinet rather than piano. On Andy Narell's mallet-tickled Jamaican steel drums, this highlighted some glassiness in their signature metallic warbling. On Nedim Nalbantoğlu's Balkan Jazz violin it zoomed in on brisk bow-on-string action - not Harbeth but Burmester in speaker terms.