On one leg or two legs?

The answer to the big question- no, hum is the same after AC plug is reversed

Just realized my meter settings were at 50, instead of 250 V
new measurements:
L amp:
Correct cheater plug 62.5 V.
Reversed 51 V.

R amp:

Correct 62.5 V.
Reversed 51 V.

new measurements:
L amp:
Correct cheater plug 62.5 V.
Reversed 51 V.

R amp:

Correct 62.5 V.
Reversed 51 V.
01-31-09: Maril555

Wow, I could see where the reading might be high in one direction, but not high in both directions.

I just checked my little ARC VT 50 power amp.

Correct plug orientation with the ground lifted was 3.6Vac

When I reversed the cheater in the receptacle outlet the voltage jumped up to 93.5Vac......
=================================

Would you do me a favor and conduct another small test for me?

Set you meter on the 250Vac scale.

Use the receptacle you were using for your test.

Insert one probe in the T slot, (neutral), of the recept and the other probe in the equipment ground making sure of a good mechanical contact.

Any voltage reading? It should be zero.

If no voltage, set the meter on the 50Vac scale and repeat the same test.
Any voltage reading? Again it should be zero.

Move the meter selector down to the next lower scale and repeat. The meter should still measure zero Vac.

One final test.
Set the meter on the 250Vac scale.
Measure the voltage between the Hot contact, small slot, and the neutral, T slot. Make note of the reading.

Next measure the voltage from the Hot contact to the equipment hole contact. Both voltage readings should be the same.
.

OK,
Neutral to chassis- 0 v. on all meter settings
Hot- Neutral 120V.
Hot to chassis:
Power cord unplugged 25 V.
Power cord plugged-in, amp. is OFF- 112 V.
Power cord plugged-in, amp. is ON- 90 V.

As usual, I have no clue, how to interpret all this.

Maril555,
You misunderstood what I wanted you to do. All the test measurements are taken at the wall receptacle outlet only. No audio equipment is involved with the test.

In simple terms the test will tell if there is a good electrical continuity between the main incoming service neutral conductor, of your electrical service, and the equipment ground. At some point the electrical service entrance neutral conductor is connected to earth and is also bonded, connected, to the main equipment enclosure as well as equipment grounding bus bar/s. In most cases this is done at the main service electrical panel. Not sure in your case though. I noticed from your pictures the main breaker is not in the main panel so it is more than likely out side by the electric meter. Connection to earth and equipment bonding may have been done there.
So the test is a simple way to look at the thing just from one branch circuit and receptacle.
I tried to explain best I can in layman terms.

So at the wall receptacle outlet only.

Measure from the neutral to the equipment ground, (U shape hole).... Should show 0 volts

Measure from the hot to the neutral.... Should be around 120V to 123V

Measure from the hot to the equipment ground, (U shape hole)... Should be the same as the voltage measurement from Hot to neutral.

And one more measurement if you would. Plug in and turn on one of the power amps and using the other receptacle of the duplex receptacle measure the voltage from the hot to the neutral. This will give a somewhat load condition on the branch circuit for any voltage drop. The voltage should be within about one volt of the reading with the power amp not connected and turned on.
A true test would be to play some music at a moderate level and take the measurement.