OP:
I’ll answer this in practical and technical ways.
The series mode protectors add noise filtering that starts at 3 kHz, right in the midrange. Other protectors can’t start till 100kHz or so. MOVs by themselves have no sound but often are complemented by additional EMI/RFI filters (again, 100kHz or higher).
So the power purist would say MOV’s are best since they have no sound and are not in the circuit until an overload but that's only looking at the coin shaped MOV itself, not the additional EMI/RFI circuits that often come with them.
For me, given I live in a lightning prone area, series mode is my standard for my audio gear. The argument that purists make that it’s in the way of the power delivery just doesn’t ring true for me. They are transparent at 60 Hz which is what you want.