I own the M20 and know the Concerto's quite well. Here in Europe, the Concerto are MUCH cheaper than the Revel, so that they are placed at different market points.
Having said that, of course i prefer the M20. Not only I prefer them, but i think they belong to a completely different league. they sound much cleaner, articulated, dynamic and accurate than the Concertos... they simply cannot be compared
You should also keep in mind that the "family sound" of the Revels is quite different from the Souns Faber, so I think you should listen by yourself (also the Signum would provide a better comparison with the M20)
The downside of the M20 is that they are much more demanding on the amplifier (and also on the quality of the front-end) than the Concertos...
Good luck!
Vermeer
Having said that, of course i prefer the M20. Not only I prefer them, but i think they belong to a completely different league. they sound much cleaner, articulated, dynamic and accurate than the Concertos... they simply cannot be compared
You should also keep in mind that the "family sound" of the Revels is quite different from the Souns Faber, so I think you should listen by yourself (also the Signum would provide a better comparison with the M20)
The downside of the M20 is that they are much more demanding on the amplifier (and also on the quality of the front-end) than the Concertos...
Good luck!
Vermeer