Why Single-Ended?

The way balanced line eliminates cable artifacts is twofold. First, ground is ignored, so the shield is not part of the sound (nor the is noise to which its exposed; in a single-ended system the shield is part of the signal path).

You cannot eliminate capacitance between wires.  Low output impedance helps to lower effect of it, but it so does with single ended design.  It will be difficult to get rid of shield to wire capacitance since many preamps have balanced output referenced to ground.  It will be pretty much any transformerless output stage including my Benchmark DAC1.

I get that it does not make sense for you. But that does not mean it does not make sense. The idea here is to remove distortion sources (now this is strictly my opinion). If you can't use feedback to get rid of distortion, how do you get rid of it? Eliminate distortion sources! A common complaint about tubes: 2nd harmonic (ask any solid state guy). OK- fully balanced differential design gets rid of the even orders. Now we are left with the odd orders. To reduce them, we set bias points in the voltage amplifier such that it cancels the odd orders. Then design the circuit to use as few stages of gain as possible (in our amps there is only one stage of gain, making them a simpler signal path than an SET). Use triodes throughout. Get rid of the output transformer (which may or may not add distortion). Take care to avoid obvious diode issues (proper metallurgy) in component selection. Stuff like that.

You forgot that we are discussing Fully balanced topology and not design of particular amp in general.  Setting bias point, as you described, can be done to single ended amp and has nothing to do with issue that we're discussing.  Please tell me how Fully balanced amp topology reduces third harmonic better than single ended amp.

So Ralph, you have come over to the dark side with an SE preamp. There is some hope :>)

Ha! We've been making some variant of that since the company was founded. Not sure that 'come over' is the right phrase...

if you transformer couple it with a center tapped secondary then that would definitely be balanced.

I very much doubt that there even is such a thing as a center-tapped SUT for the reasons I outlined earlier. We have made up SUT boxes that were entirely balanced using Jensen SUTs. All transformers (including SUTs) are capable of operating single-ended or balanced; no center tap required- works better without it.

You forgot that we are discussing Fully balanced topology and not design of particular amp in general.  Setting bias point, as you described, can be done to single ended amp and has nothing to do with issue that we're discussing.  Please tell me how Fully balanced amp topology reduces third harmonic better than single ended amp.

It doesn't. The 3rd harmonic (which is musical to the human ear) occurs at about the same level or slightly less (see below) than the 3rd shows up in a single-ended design (this is assuming open loop).

Because the 2nd is pretty well eliminated, the result is (due to the ear's masking principle) greater low level detail since there isn't a 2nd order to obscure it. That there is more low level detail in such amps is not a subtle thing- its quite easy to hear. Its nice when the measurements and the subjective experience agree.

When I was speaking of the bias point I was not referring to the output tubes, but the bias point at which the voltage amplifier is set. This is not an adjustment; its the value of resistor in the cathode circuit, which can affect the distortion and gain structure of the voltage amp. In the case of a differential amplifier, this resistor is in the CCS circuit and its value can be critical. In essence the value can have an outcome on the kind of distortion the voltage amplifier makes, particularly when it overloads. I prefer to set that value so that the stage is clipping in a symmetrical fashion. This makes for more of a 3rd harmonic rather than a 2nd, but also can have the effect of reducing the amount of the 3rd harmonic.

You cannot eliminate capacitance between wires.  Low output impedance helps to lower effect of it, but it so does with single ended design.  It will be difficult to get rid of shield to wire capacitance since many preamps have balanced output referenced to ground.

I hope you are not suggesting that single-ended cables can run as far as balanced... I'm going to refer you to Wikipedia on this one.
https://en.wikipedia.org/wiki/Balanced_line

I also refer you to AES file 48 which I've mentioned several times. The Rane site has a great little article that should help clear things up:
http://www.rane.com/note110.html

The final statement in the quote above points to the possibility that you did not read my opening post in this thread. I will therefore repeat myself in saying that many high end audio products don't support the balanced standard, such as the preamps to which you are referring in that last sentence.

When the balanced line standard is not supported the interconnect cables become audible and then audiophile tend to spend a lot of time and money trying to eliminate the audible artifacts that result (IMO/IME its simply easier to just support the balanced standard). This is the reason why this debate is still around (as Herman points out) years on after we introduced balanced line to high end audio.

To Ralph's point about the audibility of cables; I settled on DIY CAT 5 cables as interconnects and speaker cable years ago. They are neutral configured as single ended over any practical length in the home. I encourage anyone who is on the cable merry go round to do the same and focus on things that have more impact. If you are a million dollars into your system and want to play around with mega buck cables then have fun. If you have a budget like me then cables are the last thing to spend any money on. IMHO playing with the placement of your speakers will have much more impact than cables and it is free. On the other hand, you can go with Ralph's approach and eliminate cables from the equation

However, as a DIY guy that builds his own SE amps I have the luxury of tuning the sound to my preference with the active circuits and don't have to feed the coffers of ridiculously overpriced cable makers. It is inconceivable to me that someone would pay $20,000 + for speaker cables. That tells me 2 things; PT Barnum was right and I'm in the wrong business

Honestly you need to audition all kinds of amplifiers (I have). They work best with different speakers as well. I personally don't like SET amps at all (and I've owned 101db and now 91db speakers and tried at least a half dozen of them). I tend to prefer 3rd harmonic over 2nd I guess. But the people who are adamant that only one way is technically correct or sounds right are to be ignored. 

Ironically, the best I've ever heard Avantgarde was Trios w/ basshorns on BAT tubes years ago. The audience gave standing ovations - but darn those complicated balanced amps :)