This is his answer:
Hi Magnus,
The stock Stax supply is nothing but a transformer with a rectifier
inside so nothing special. Feed it 230V and it will give you raw
12VDC. Now if you are using a switching PSU then that could upset the
switcher in the amp if it runs at the wrong frequency and the sound
will suffer. It could also be struggling with the load for some reason or
a bad component in there.
Hard to say.
My PSU is just a basic switcher that runs at the right frequency to
work with the Stax stuff and not supposed to be an upgrade in any way.
I had to look up the ifi stuff and yeah...audiophile snake oil BS.
"Military Radar Tech for Audio"... what a pile of rubbish. Neat package
but that's it.
Now what I do for my own amps is to go through old routers/switches
(stuff like that) and search for 12V/1A PSU's and then switch the polarity
on them.
Some of these are just transformers and rectifiers in small brick and
work perfectly for Stax once the polarity has been swapped.
So how I interpreted it, the PSU won't make a big difference if my current one is working properly. Even so, for the price he's asking it's a small risk, and I'm planning on keeping the amp for a while. So I've decided to buy a PSU from him