W4S Mamp or Bel Canton REF500 m ?

I think the W4S would be a much better match for your speakers, and the kind of music you listen to.

I have both - I prefer the REF500Ms. The difference is subtle but it is there.

The noise floor is lower on the Bel Cantos and it seems more effortless in driving the speakers.

The price for the Bel Canto is x2 than W4S.
So the real question is : is it justify ?
Better Sound ? Better design ?

When i look inside, the both look like very different, but in the same time they used the same B&O module...

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.