AC Power - two circuits better than one?

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.
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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?