'Ground Tester'....
Are you referring to the 3 or 4 pole fall of potential ground test used to evaluate the effectiveness of an earth grounding system?
AEMC Instruments model 4620 ground resistance testers | Electro-Meters
Yes. For me, that's THE starting point. Check if the groundpin(s) do effectively sink
current into the earth.
Over here, a lot of groundpins in sandy soil run dry in the summer and don't work. Not a problem in new buidlings, but often a problem in older ones. Stuff works well when it's wet, but gets spooky when it's dry.
'Megger'....
Insulation tester that applies 50 or higher vdc to a circuit under test, either to ground or between conductors and/or shield of a cable to determine electrical leakage. Maybe of some value if wetness of internal house wiring is suspected, but to do properly the electrician would need to ensure every electrical device is isolated away from what is being tested. Not worth the effort in my view.
Well worth the effort in very old buildings. Sometimes leakage occurs in the wiring. But it's just one of the tests if it isn't clear from the beginning what the cause is.
I've used the two testers above somewhat frequently through about 29 years of a career with an electrical utility, so I have a pretty fair familiarity with what they are used for.
They only come out once or twice a year. But when you need them, they are irreplaceable. Mine is a combined one.
I do use a DMM if I don't have one on hand, but that gives only an impression. And that impression can be completely wrong. It also produces mysteries. Over here, most mains nets are symmetrical. If you measure mains between one wire and ground, you should have half voltage. We've got 230 VAC; so you should measure 115 VAC to ground. I have one outlet in my house that measures 140 VAC. In theory, there should be 280 VAC on the wires. Of course, there isn't...
My guess on the OP's issue is much of his 'studio' gear is powered from 2 pronged wall warts which may not be well grounded and there is some capacitive coupling of the AC into the studio gear. A digital multimeter could be used to see if there is AC between the guitar amp and the studio gear, but I would probably use an old analog Simpson type meter to start as it would tend to load down capacitive coupled voltage more than a DMM. Usually switching a Simpson to a lower voltage range will reduce the voltage if capacitively coupled.
I know enough about mains in other countries to be extremely cautious. I lost a dear friend to electrocution...
Wouldn't be a bad idea to get one of these testers to ensure the outlets the gear is plugged into are properly wired (Walmart, Lowes, or Home Depot type stores will have these)
Power Gear 3 Wire Receptacle Tester, Outlet Tester, 6 Visual Indications, Light Indicator, UL Listed, Yellow, 50542 - Voltage Testers - Amazon.com
I hate those. Just like a DMM, they lead to wrong conclusions in rare cases. And that's when it becomes very dangerous. And we even threaten them with reporting to the authorities.
I do trouble digital/analog shooting. But repairs in ground installations are always left to qualified technicians from one company. We know they don't cut corners. And they know we test again after repair. If a customer refuses repairs, we don't consider them customers any more.
I don't mind patching an audio cable with tape, if there's no time to solder.
That's something I will NEVER do with mains.
My friend who got electrocuted is dead. He wasn't even on the ground, he was up in the rigging, checking cables. It was a festival, so an outside rig and someone had decided to disconnect the proper power from the distribution box and connect it to a very long cable going to a standard wall socket in a nearby bar. That was enough to power the drill and a few pieces of gear. The main PA wasn't installed yet. How was he to know there wasn't a ground connection in that one wall socket?
We have strong rules for electricity. I follow those, even when some people consider me over-cautious.