Do amps have a sweet spot?

In most any practical sense, yes, it may be normal, especially for lower-priced gear, to have that portion of its volume range that seems to open up a bit better...although exactly which portion that may be could possibly be different in different systems. In one system the amp could sound better at 3/4 of its output and yet in another system sound better at 1/2. Usually when you start moving up the food chain and start buying better and better components, this sort of thing becomes less and less apparent and vol. and dynamics start to become more "linear" (as do power supplies). By the time you've done that, and/or have gone successfully enough down the power-conditioning road and perhaps have also looked into things like how much your vol. control is veiling the sound, being somewhat picky about interconnects for your own app and even about things like grounding issues, what you can end up with is an extraordinarily linear rig...although all that typically comes with the price tag as well, of course. But, if you did all that, the I believe the surprising part to me is that even your current gear would pass that kind of test with flying colors, giving absolutely ideal performance, no matter what the vol., right up to clipping. So the real answer is that, yes, you do have any given amp's tendency to sound better at whatever point it will, but, that characteristic is in no way inherent to the amp itself, and, in the long run at any rate, there are those things I mention that you can look into to improve the situation. All this that I'm talking about here is apart from the ins and outs of matching your amp to the right speakers and so forth, which could at least influence things. Hope this helps.