OK I found this from a search:Here are two more measurements of the quiet noise at the amp output with another linear 12v power supply.
(unmodified (500xgain) SRM252 unit, input poti at minimum, driving SR003). Worse peaks even between 3 and 4.5 kHz.
Shows a relative big influence of the wallwart psu at the amp outputs.
https://orronoco.blogspot.com/2019/10/simple-component-upgrade-stax-srm-252s_3.html?m=1
You might need to translate it via browser but these are interesting mods and the guy appears to know the circuit well and provides schematics.
GDNW