If the OP’s power amplifier’s power supply was not heavily over regulated, and maybe even then, the circuit that feeds the amp is staving the amp of power when playing high dynamic music material. The dimming lights is proof of that. The AC mains VD may be, (probably is), lowering the amps rated power output.
An an oversized high capacity shielded toroid power transformer is meaningless if the AC mains voltage can’t feed the transformer the power it needs to operate properly.
Quote from link provided by OP.
Power Supply Circuits
The Vision SET 120 Control Amplifier has an oversized high capacity shielded toroid power transformer and multiple regulated power supplies for all areas of the amplifier including the output transistors, to provide incredible dynamic range, bass extension, transient detail, and air. All of the power supply parts are selected for long life, reliability and operate well within their technical specification ratings..