Attenuation vs. 'Gain Control'

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Nidri

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Over the years I have occasionally read about certain amps that control volume by controlling gain, as opposed to attenuation (which I understand as signal reduction).
E.g. the PS Audio Gain Cell products from a couple of years ago, as well as more recent Ayre models.
(I'm sure there are other examples.)

As described in Stereophile (bear with me):

* "Hansen's other remarkable idea for the AX-5 is a circuit innovation called variable-gain transconductance or VGT, first seen in Ayre's top-of-the-line KX-R preamplifier of 2008 ($18,500). As Hansen explains, most active preamplifiers work by applying to the input signal a certain amount of voltage gain, so the signal can effectively drive a power amplifier. But in order for there to be a reasonable volume range?and to simply keep the playback level from being too loud?the voltage-gain stage is preceded by a potentiometer, which attenuates the signal. The drawback of this is that such a preamp will exhibit its maximal signal/noise ratio only at its maximal (unattenuated) volume. As Hansen puts it, "Since most preamps are used anywhere between ?10dB and ?40dB for an average listening level, this means the S/N ratio in actual use will be 10?40dB worse than on the spec sheet. As implemented in the AX-5?which doesn't incorporate a preamplifier stage per se?Ayre's VGT circuit allows the user to determine how much gain is generated by the amplifier's input stage, which itself comprises a total of four complementary-differential JFETs. The volume knob on the AX-5's front panel controls a pair of enormous, motor-driven, Shallco silver-contact rotary switches, each of which contains dozens of hand-selected, low-noise resistors. Every volume-level adjustment made by the user has the effect of switching into the AX-5's input-stage circuit a different set of resistors, the values of which alter the transconductance of those JFETs?and thus calls into play a specific level of gain corresponding with that setting. The volume system has 46 steps of 1.5dB each, over a range of 67.5dB. (I'm told that, by changing a single resistor in each of the AX-5's channels, one can adjust the overall gain range to accommodate, say, speakers that are significantly more or less sensitive than average.) Thus the AX-5 doesn't use signal attenuation at all, but rather creates variable input-circuit gain, on demand, to suit the desired volume level."

Sounds sensible to my non-engineering brain.
Or is it unnecessary over-engineering - a solution in search of a problem?

[ DETOUR ]
Then, in the Stereophile review of the Rotel RB-1090, I read that the amp had lower distortion at 2/3 power than at 1/3 power. +
Why would this be?
Do amps sound better when driven harder?
Does attenuation 'throttle the beast'?
Let's say you have a high-gain preamp with a high-gain power amp. Somewhere along the chain you're going to need an attenuator to drastically reduce the signal strength.
Is that not like driving a car flat-out but with one foot on the brakes at the same time?

Would the purest system of all be one without any volume control at all, i.e. a set amount of gain?
Not practical, of course, but if you were a crazy audio guy and always listened to the same album at the same level (like I said, crazy) then such a system would surely do the trick.

Is there just too much gain in audio systems in general?
I'm dumb-founded that, in my bedroom system, music plays (softly) even with the volume on the preamp turned all the way down, i.e. at 'zero'. Preamp has 0dB (single-ended) / 6dB (balanced) Gain and power amplifier 35.1dB/29.1dB Gain, 0.5Vrms/1.0Vrms Input Sensitivity. 
For that matter, do volume control pots have a lesser effect higher up in their range?

Not looking to start any kind of debate, just curious as to why certain approaches are chosen/preferred over others.

* https://www.stereophile.com/content/ayre-acoustics-ax-5-integrated-amplifier
+ https://www.stereophile.com/content/rotel-rb-1090-power-amplifier-measurements
 
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