My friend was using a Mac 2102 amp so I don't think there's any problem with its power supply or its design.
This is an all tube design resurrected from the 60's.
Sam Tellig in a 2001 Stereophile review says,
I hinted to Larry Fish that I'd heard a subjectively more powerful sound with the MC2000. The MC2102 has a smaller power supply, and its output tubes are run at lower voltages. The MC2000's plate-supply voltage was 500V, the MC2102's is 450V. Hence, 100Wpc instead of 135Wpc.
and
If I'd been immediately impressed by the MC2000the detail, the definition, the vividness of the sound, the dynamicsI was less impressedless blown away, at firstby the MC2102. I heard a less powerful, less dramatic amplifier, even though there's only a slight drop in power from the MC2000from 135Wpc to 100Wpc. Bass was tighter with the MC2000, if memory serves me right. There seemed to be more dynamic headroom. None of this was surprising, considering the MC2000's beefier power supply and two power-supply transformers. The bigger amp simply produced a bigger sound: a deeper, wider soundstage and better dynamics. But I began to warm to the gentler, less immediately impressive, possibly less insistent sound of the MC2102.
I suggest that you may be running into amplifier power supply issues...especially if you are pushing the amplifier anywhere close to clipping. (In this case, a few volts drop across a power conditioner might make an audible difference in "dynamics" that you and your friend heard. When stuff gets close to clipping you get odd effects, as even slight issues or differences may become magnified in the distorted presentation)