I believe they are more efficient in most circumstances... but they do not have "headroom" - pretty similar to class-D amps!
From my reading (I am no expert) - a linear power supply with big iron transformer, can provide short peaks of output above its continuous specs, which provides some headroom for musical peaks potentially - with SMPS, the max out is the max out, there is no headroom - so if the design was in any way dependent on headroom, replacing linear with SMPS, requires "oversizing", to provide the headroom. (the end result will no doubt be an improvement, as a result)
It would be nice to be able to take a vintage Quad 606, and replace the trannie with an SMPS having a higher current rating... current was always the 606's one weakness IMO.
From my reading (I am no expert) - a linear power supply with big iron transformer, can provide short peaks of output above its continuous specs, which provides some headroom for musical peaks potentially - with SMPS, the max out is the max out, there is no headroom - so if the design was in any way dependent on headroom, replacing linear with SMPS, requires "oversizing", to provide the headroom. (the end result will no doubt be an improvement, as a result)
It would be nice to be able to take a vintage Quad 606, and replace the trannie with an SMPS having a higher current rating... current was always the 606's one weakness IMO.