That’s good. It doesn’t mention that in the manual because we ship the PS-2 and PS-100 with the line level control all the way up.
There’s no hard rule on the front panel level switch. It’s intended to make the PS volume convenient to use.
If your amp is 50 to 100W and cranked, you’re more likely to have that button pushed in so you don’t have to deal with an overly sensitive volume control that goes to 11 at around 8-9:00.
Under most normal circumstances though, just leave the button out.
Cheers,
Dave