Working under the guise of my pretension of posting only when Safety is involved.....
Several points to ponder:
a) Yes, working on some (most!) amplifiers can be hazardous. Very small solid-state things like frets ' single-chip amp won't be much of an issue, but even that chip, in come circuits, can present voltages that might make you jump, so a word-to-the-wise, and all that.
b) Today, 99.99% of all gloves are not made of rubber, but of some kind of plastic. I'm sure that they still provide some protection, but will it be enough in a high-voltage situation, that's the question.
c) Keeping one hand out of the amp, period, is not a dumb idea. It may be impractical in some instances, but for the most part, you can clip the meter's Common lead to the chassis, and probe the circuit with the Hot lead with just one hand. The "left hand" (I take exception to that, I'm ambidextrous) can be anywhere that's safely away from the amp.
d) No, most techs don't wear anything special, but..... They also learn to take off their rings, and possibly their wristwatches. Arcing (that's when a current flow jumps across an air gap and makes contact, usually through your body parts) can be just as devastating as a direct contact, and metal jewelry only exacerbates the problem. I've never suffered such a malady, but I have seen what happens when someone connects with approximately 1,000 volts of RF from a 250 watt transmitter - he broke through the two desks behind him, and broke his back into the bargain. Oh, and he lost two fingers of his left hand.... guess which two; was it perhaps the one that fell to the floor with a melted lump of metal, do ya think?
e) It's been 10 years and more since our illustrious Mentor Emeritus departed these halls, so I've taken it upon myself to carry the flag, and to repeat his mantra - here it is:
Someone is always the fuse - make damn sure it ain't you!
Rule #1: All Lives Are Final. Make sure that the life you have just been issued is appropriate for your needs, before departing the womb.
Rule #2: In case you don't like the life you have, see Rule #1.