Friends, family, dogs, cats, the cleaning lady. Passing birds
I think I mentioned most items in this list before, but you definitely bet me with "passing birds".

Now I have those pctures in my head.
I don't see how given it's an analog, manual, rotary volume control.
At least in theory, there could be electronic switches bypassing the volume pot, which cannot be used, yet, due to missing app support. Doesn't sound all that likely to me.
In any case thus would only make sense for a complete bypass, so full gain, and some are not happy with that thought. Implementing yet another stepped volume control this way was definitely pointless.
From the ASR measurements we can see that both Amp and Amp Pro were designed so that they cannot reach 1% THD at all.
Exactly. So, no matter how you put it, the Vibelink Amp does have a higher output power, indeed.
It may feel a bit like cheating, but technically it is not. The Vibelink Amp can get (a little) louder, not only on average, but also for peaks. And this is not the only difference, of course. Pretty much all measurements published by Amir at ASR look better than those for the Amp Pro.
There's no 100% match between use cases, of course.