Sounds and awful lot like the previous amp was cutting you off at the knees and routing the lower frequencies out to the sub. it might have been burried in the menu somewhere and so the roundoff of low frequency that the speakers were capable of was not getting sent.

Anthem: AVM60, Fosi DAC-Q5
Axiom: ADA1500, LFR1100 Actiive, QS8, EP500, M3, M3comp, M5