There is no polarity on the adapter because it is a 9VAC adapter. The party line is, the RP Series are meant to work with the 9VAC PS0913B-120 (or 230, 240 for EU markets) and not 9VDC adapters. Those universal power supplies almost never work and can actually damage the pedal.
The only other adapters we have seen work with our multi FX units are the Line 6 PX-2.
Other after market non OEM power supplies are not supported or recommended.