I think this all makes more sense now. Because it was so loud already on low gain I didn't go much above the first ten steps or so on the volume control, and thought the rest of the range would be really extreme (not to mention high gain). Now I've tried some different sources (all pretty bad, phones, laptops) and played with the volume control on those, and now I can see that the rest of the volume range on the SA-31 isn't as extreme as I first thought. The difference between high gain and low gain isn't very large either. Those 10 dB for me translate to 6/79 on low gain being equal to 1/79 on high gain, more or less.
So, now I'm ready to enjoy the amp fully without worrying. It really is a step above the M-Stage, as it should be, and as reported it's not quite as warm even with two jumpers. To me it seems very detailed, dynamic, spacious, dark in some sense - if the M-Stage was a warm and comfy chic-flick this feels more like The Dark Knight Rises. ;-)