Not 100% sure, but I believe the XPA2 has the same waveform limiter circuitry as the MPS1, so that would make sense why it does not like to drive 4ohm speakers.

Since I sent back my MPS1 and purchased my Odyssey monoblocks the 80's have never been more happy. \:\)

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85