That is really strange, open it and see whether there is anything loose and look funny.
I google to try to find the schematic, the closest is the HCA-1000 schematic. Maybe it is close to yours. There is really nothing in the circuit controlling the frequency response. The circuit is very typical power amp with complementary input stage and 4 output transistor pair. Maybe yours have 2 pairs instead as yours is lower power. It is very similar to my Acurus except this one use JFET transistor as input. It has some DC servo control on this one.
It is most strange that you have scooped mids rather than highs rolling off. The only part in the circuit is part I circled in red. But those affect frequency at over 100KHz for stability compensation.
If you have the schematic, post it so I can look at it. Don't take for granted HCA-1000 is the same with just more output transistors.
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