Lordathestrings, I got a tester exactly like the one you have in the picture. I first tested it on a (new) powerstrip linked to another- got an "open ground" message on the LEDs. I turned the PS on an off, then test all the outlets- they tested fine.

There's no way I can test the amp personally, so I'm still running on battery power. I personally think while do the setup, I may have dislodged a wire in my old Strat, or something was up with the loose sheath on the guitar cable. I really don't think it's the amp- or VERY unlikely.

So, I'll play it safe for now. Thanks much for your guidance.