Yeah, i`m kinda interested in this topic. I have a bunch of those external hard-drive psu`s (line lumps) that seem to give plentiful 12v and 5v. But wondering how applicable these psu`s are to audio-usage.
One answer is to find a common mode choke, put that in series with both the +5V and GND terminals, and then put a shunt cap system after that. This will significantly reduce power supply noise, and add little ESR that could reduce your 'end of the day' supply voltage.
Agreed. This is key. A good size high mH common mode choke will make a huge difference in SMPS noise. Maybe if the SMPS is old or just poorly designed there might be some low frequency noise. But with a regulator after it, it will be difficult to see a difference with a linear supply.