Very interesting, and educational, thanks (and nice test gear, beyond my means). I hadn't thought at all about harmonics from the rectifier switching (and I don't think I've ever come across it in anything I've read).HP 6632B precision power supply + Keysight 33465A, in case you're interested.
I'd be interested to see the circuit diagram for the PSU, and the location of the fuse.
A simple, 'classic' linear supply has an absolutely horrible V-I characteristic on the AC side. The current is zero most of the time, but as soon as the peak of the mains waveform exceeds the voltage on the reservoir cap + diode drop, the rectifier diodes turn on resulting in a huge dI/dt. A large current then flows into the caps until the mains waveform reaches its peak and starts to reduce, at which point the rectifiers turn off and current drops to zero again.
The resulting current waveform is a series of spikes at 100 Hz intervals, producing electrical and magnetic interference with a Fourier spectrum of harmonics that extends throughout the entire audio band. (Think about that the next time someone suggests that linear supplies are "quiet" in a hi-fi context).
The argument for the effect of the fuse could go either way. You could argue that, by having resistance, it's slightly reducing the magnitude of the current peak each time the reservoir caps are topped up, resulting in less interference in the audio band. In this case, more would be better, and you might expect the amp's power supply to include a series resistor for this reason.
Or, you could argue that the voltage drop across the fuse is modulated by the current being drawn by the amp, which in turn follows the audio signal, and that this is 'bad' for some reason that almost certainly depends on the specifics of the amp's design. In that case, lower resistance would be better.
PSU schematic attached, plus block diagram showing fuse F1 at bottom left. Only difference is that the 10,000uF caps have been replaced with 15,000uF ones as part of the recap. There's also a pair of 4.7uF (bypass, I presume) caps on the +44V and -44V lines at the output transistors.