AC Power - two circuits better than one?

The one thing to avoid is to use both legs of the incoming 240V. To check for this (if using two separate wires from the breaker box, is to measure the A/C voltage from BOTH of the hot wires from the two outlets.
So you would have either:
Checking all four wires against each other:
#1 with #2 120V ... #3 with #2 120V
#3 with #4 120V ... #1 with #4 120V
#1 with #3 zero voltage ... #2 with #4 zero voltage
OR you might have one pair measuring 240V even though all the other measurements are 120 or zero.
IF you have a 240V in there with a possible combination.. That is bad. Most likely nothing will happen, BUT if one of you components develops a short, or is miswired, you could have a 240 volt surge through your stuff, both ruining a big pile of equipment, and hurting yourself.
So avoid two different legs of the incoming A/C.
Using an outlet with NO other stuff on it is the best. The 20 amp could be better if you have enough of a load that you are usingg a lot of a 15 amp curcuit. (If the A/C cord at the wall is HOT, you need a better cord, or bigger capacity wall amperage.
I have a ton of stuff on a 15 curcuit, and find it is OK. I used to use a 20 amp one, but that entailed running a ten guage extension cord 100 ft, and I decided to just stop using that.
With two monoblocks, I would use the 20 amp single..
(Another problem that could arise from using multiple outlets is hum from a differential in the grounding of the two outlets.)
But, if you have two 15 amps on the same leg, then try both ways and see.
(When I say the same leg, I do not mean the same wires from the breaker box, it is two separate sets of wires from the breaker, just avoid using two that are on different legs of the incoming 240 volts)
IF you do not have a voltmeter, you can cheat (but be careful!) wiring up a small 25 watt light bulb. if you have a 240v the light will burn out rather spectacularly with a big flash when it is connected to the 240 V.
If you are on the same leg, the bulb may light, or just not do so.

Thanks for the advice Elizabeth, but I still am not quite sure how to check that the circuits are on different legs. I should be checking that the voltage on any two of the circuits doesn't total 240V? I'm a little confused by how I would actually do that - measure one at a time or two simultaneously?

Salmonsc,

If the circuits are fed from the same leg you will measure zero volts from the hot slot contact of one branch circuit receptacle to the hot slot contact of another branch circuit receptacle. If you measure 240V between the two measured branch circuit hots, they are fed from opposite legs.

I doubt you will find that all 3 branch circuits are fed from the same leg. Might be though.... Good chance 2 will be. Use the dedicated 20 amp branch circuit as the starting reference point. Measure from the hot of the 20 amp dedicated circuit to each hot of the two convenience 15 amp branch circuits.

You can use a drop cord to extend one lead of the volt meter. Note the smaller length slot of the two slots is the hot conductor. You can verify by measuring from the smaller slot (hot) to the equipment ground (round hole) contact. You should measure 120V.
.

Jea48 said it MUCH better than I did.
(sorry)

Many thanks Elizabeth & Jea48, I will check this out tomorrow. Assuming I can find a second branch circuit fed from the same leg as the 20 amp circuit, I will likely connect the monoblocks (& possibly the power amp) to the 20 amp circuit & the sources to a 15 amp circuit. Does this make sense?