W4S Mamp or Bel Canton REF500 m ?

Bel Canto initial price is somewhat high, but used units have great value and yes they're better sounding amps.

Does the Mamp line do anything special with the power supply?

That used to be the thing that distinguished the BC Ref500m and ref1000m from similar Wyred offerings.

Other than that they both use similar Icepower amp boards and add their own input circuits to mate beter with higher output impedance tube pre-amps so not much to distinguish the two otherwise on paper.

The REF500M has a power supply board that cleans the power between the AC and the ICE module. The mAmp doesn't.

I had the original REF1000 which was upgraded to the REF1000M - with the addition of the power supply board and the improvements to the noise floor was noticeable.

REF1000 was stock Icepower in a box. Ref1000m added power supply board and input circuit board for higher input impedance to match better to tube preamps. Wyred added similar input circuit board as well only last I checked.

So the differences vary between various BC models and Wyred with original BC amps being most basic Icepower, Wyred the next most refined and m series BC amps the most refined over base level Icepower.

Relative prices reflect this hierarchy in regards to technical level of refinement over base level Icepower.

One would expect a general corresponding difference in levels of performance possible however that could vary case by case depending on exact system used in.