Turns out, "normal"!! The following is a reply from the company - and I can believe it, given the response. Here is the official word (personal names omitted, but this is from a high level exec at MF ). In my view this does in fact resolve to my satisfaction this inquiry.
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Firstly, I am sorry that you are less than thrilled with your new purchase. I hope that I can explain, to your satisfaction, the effect you are hearing and why, in normal use, we don’t regard this as a problem.
Technetron is, in this very particular matter, incorrect when they say that the unit is at fault. Many of our current units will exhibit this same issue although, I must add, you are the first person who has noticed it to the extent of regarding it as a problem.
On older device, yes, we have occasionally had issue with noisy pots. This is despite us only using high quality ALPS units sourced directly from the manufacturer and rejecting units that were noisy during testing. However, over time every potentiometer will degrade. Making matters worse, left and right tracks in the pot would degrade at different rates leading to channel imbalance. This led us look for a better solution and, we believe, we found one.
We now use a device from Texas Instruments, under their Burr Brown label, to implement a digital control of the analogue output volume. This allows us to control the volume with perfect repeatability, perfect channel matching and virtually no noise. To this we then add a traditional volume control to give the customer the feel of a traditional system even though there is no direct link between the pot and the signal.
Your testing here has been very thorough and you have in fact come very close to understanding what is happening. You have observed that only sine wave signals are effected and not square wave ones. The problem, quite simply, is that the volume control chip is making adjustments in a step-wise fashion, almost instantaneously. This will mean that if an upward shift in volume occurs during the downwards part of the cycle or a downwards shift during the upwards part of the cycle then a very low magnitude, but very sharp, discontinuity will occur in the waveform. This is the noise you are hearing. It is noticeable on a sine wave because this signal is always varying whereas your square wave voltage remains at one of 2 constant values throughout almost its entire cycle. It is unnoticeable in normal use because of the complex and varying nature of the music signal.
What I can tell you is that, based upon your descriptions, the unit is working correctly.
I can also say that over the coming decades you will not experience a noisy pot impacting upon your listening pleasure with this amplifier.
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