LPS is not necessary for Purifi or Benchmark AHB2 power amplifiers.
Their superior designs use SMPS and perform much better than anything else under $100,000.
Using SMPS allows Purifi to be very efficient (94%) and so sound is produced rather than heat. The cost, size and weight is reduced.
No noisy heat extraction is needed as most of the small amount of heat generated can be absorbed by the aluminium enclosure.
Essentially its a bit of a myth that SMPS supplies are a problem for audio.
With linear supplies you can guarantee that there will be noise generated at the mains frequency and multiples thereof; 50Hz, 100Hz, 150Hz / 60Hz, 120Hz, 180Hz etc which is right in the audio band. In problem amps you will even see these harmonics stretching into several kHz range. You also have the transformer emitting an electromagnetic field which can be picked up in surrounding circuitry or even adjacent audio components. Also due to the transformers, they are very heavy. They are also inefficient, wasting electricity and producing heat.
SMPS on the other hand is light and efficient as @mboddy has mentioned. They switch typically around 100kHz which is way, way above the audio range. As such the noise they produce is not a problem. This is proven by the measurements of Purifi amps which show exceptionally low noise floors.
Why do people change PSUs on other components? Well unfortunately its often for no good reason and any "improvements" they hear is often simply placebo.
Now having said this, there are scenarios where SMPS can aggravate ground loop issue because they have what are call "Y" capacitors which leak noise currents in to the ground. These are used to improve EMC. In single ended (RCA) connected systems this can end up in the audio shield which is also the signal low. worst case is it can cause audible noise.
However i need to strongly emphasise that this is a fundamental flaw of single ended RCA connected systems. They are flawed because they use the signal ground as a shield wire connecting components together. This is a really dumb idea . Balanced XLR solves this as the shield is completely separate from the audio signal wires.