Perhaps the single biggest reason to use a switch mode power supply (SMPS) on a power amplifier is that the output of a properly designed one is more like perfect DC than that of a conventional supply; and not just by a little bit.
Conventional supplies switch back and fourth between plus and minus only 60 times a second and that leaves big holes in the DC that the power supply capacitors have to fill up as best they can. The bigger the power supply capacitors, the smaller the holes, but it's nearly impossible to fill them entirely because it takes exponentially more capacitance to get even close. To make matters worse, most high end audio amp designers will tell you that increasing the power supply capacitance too high will start to affect the sound quality of the amp; slowing it down and sounding muddled and overly dark. Indeed, there is a perfect balance in a power supply between the amount of capacitors used and the type of capacitors used.
No doubt you've seen manufacturers boast about their low esr (equivalentseries resistance) power caps - Black Gates among the most famous brands - and how they are bypassed with fast acting film capacitors etc. All these tricks of the trade do make a difference and the reason they do is because of the importance of the power supply. But I would contend that everything you do to a conventional power supply to make it sound better is akin to perfuming a pig; indeed it smells better but it's still a pig.
A properly designed SMPS, on the other hand, switches back and fourth 100,000 times a second or even faster and filling in the resulting DC holes is rather a simple matter at those frequencies. What you get out of the SMPS is near perfect DC, which helps any power amplifier sound its best.
Why haven't these supplies been used to better advantage in the past? The answer is rather simple - most designs caused more problems than they solved. Designing a SMPS is WAY beyond the skill of most high end audio designers. My next door neighbor Paul designs SMPS's for a living and he's one of a very elite group of talented "magicians" that can design and build low noise SMPS that actually outperform in every way a conventional supply.
What this means is that in-house designs of conventional supplies are trivial and in-house designs of SMPS for high end audio are nearly non-existent; so difficult the art of design for these is. Therefore, all high end audio designers have had to rely on what's available to them off-the-shelf and those have not been designed, necessarily, with high end audio performance in mind.
Tomorrow we'll see what's different.