Without knowing the AMP's design: We do know they are using a digital amp. Many digital amps switch the rail voltage depending on the load.

If I had to take a guess... I would say the AMP is incorrectly switching the rail voltage low, for a low impedance load. That would perfectly explain why it seems you are only getting a fraction of the power you normally would expect. Because you literally are (25% of normal power depending on what ohm the speakers are).