Look at the specs of the PuriFi modules, which can put out 500 watts into 2 ohms with 0.0003% THD.Look at what independent tested current it can do.
Compare the "2ohm wattage" to the "4ohm wattage", and see if the 2ohm can come "close" to doubling the 4ohm, if so there’s no current limiting.
If it’s similar or lower, then there is severe current limiting/sagging.
Cheers George