I was right there with you ... and then you mentioned 'isolated ground.' Now we have another complication to address.
First, the 4-squares would work fine; I;m not entirely sure that isolated ground versions of the receptacles exist, though.
Your saying "208V" makes me think of three phase systems. If there are to also be 120V circuits, you will want to avoid getting the neutrals combined.
If the receptacles are isolated ground, that leaves open to question just how the cases of the amplifiers are grounded. I doubt that the amplifiers qualigy as 'double insulated.' Therefor, in addition to the power circuits, you need to ensure that the rack, and the cases, are reliably grounded.
