One thing I should add about these amplifiers. The Sunvalley, and I presume the Sun Audio, are Japanese appliances that are not grounded in the same way that that US appliances are grounded. They use 2-prong power inlets and power cords instead of our standard 3-prong power inlets and cords. So I understand it, it's because Japanese engineers approach safety measures differently, focusing more on preventing short circuits than ground issues.
However you feel about the relative safety of floating grounds, they can be noisy, susceptible to electromagnetic interference. And indeed my unit was, probably because I set it in a spot in the house near all our wifi/router gear. Because the soft, static noise I was hearing didn't go away completely when I turned the volume all the way down, it was clear I had a grounding problem. After checking all my ground connections for cold solders and bad connections, I decided to earth-ground the unit, which was quite easy to do.
Anyone who decides to build the Sunvalley will see in the build guide that all the ground wires connect to a central ground which is, not surprisingly, a through-the-case screw by the rectifier that holds down one of the terminal strips (it's marked "ground point" on page 8 of the illustrated wiring guide). That screw simply needs to be connected by wire to the third prong of a replacement US-style power inlet. Happily, those inlets very easy to acquire, indeed the product linked below is an exact duplicate of the two-prong inlet that comes with the Sunvalley. The screw holes even match up.
All you need to do is install the inlet, connect the internal power leads to the usual spots, then connect the inlet's third-prong ground to the ground point (the US version even includes wire and spades if you want.) The process takes about five minutes.
Anyway, earth-grounding the unit solved my noise problem, and I expect it would solve similar problems for other Japanese amplifiers. Plus in the end you have, at least IMO, a safer amp. Here is the product I mentioned: