I have a Roland Cube 15 guitar amp that hisses. It does it regardless of what is plugged in (or not plugged in, as it will do it when the guitar and its cord are unplugged from the amp) or what the dials are set to. When I first turn it on, it gives a buzz for a few seconds then switches to the hiss. I opened it up, nothing looked wrong (check the tops of all the caps) and re-flowed the solder on the big filter caps to no avail.

I did find the schematics online here if that helps:
http://schems.com/bmampscom/roland/Roland_Cube_15-30_sch.pdf

I would appreciate any insight! thanks

 

My thought it is a limitation of the TDA2025 power amp IC. Unless you change the PA for a low noise circuit, you are just going to have to put up with it.
There is a thread here that may, or may not help.
I've not read it all.
https://www.diyaudio.com/forums/chip-amps/126969-tea2025-alternatives.html

 

The "mute function is directly ahead if the TDA2050 power amp IC. If it hisses when muted then the TDA2050 is causing it, but a TDA2050 is obsolete and is not made anymore.