After studying the amp a little closer my guess is it's a problem with the switching input jack, it switches the preamp out when you don't plug a cable in.
Unplugged state : input is wired direct to the power amp (it seems), certainly to prevent noise from the preamp when nothing is plugged in.
Plugged state : input is wired to the preamp, but it seems the path to the power amp is still ON, and it should be OFF. So that create a parallel signal. The signal leaking through is perfectly clear tho, like it's the full signal from the pickups into the power amp.
If it's as I guessed, it's an easy fix. But there might be something else going on, I won't know until I open it up.
The input part of the amp seems fishy anyway, as it doesn't properly load the pickups and brings a lot of noise on the Hi input and a lot less on the Lo input.
Unplugged state : input is wired direct to the power amp (it seems), certainly to prevent noise from the preamp when nothing is plugged in.
Plugged state : input is wired to the preamp, but it seems the path to the power amp is still ON, and it should be OFF. So that create a parallel signal. The signal leaking through is perfectly clear tho, like it's the full signal from the pickups into the power amp.
If it's as I guessed, it's an easy fix. But there might be something else going on, I won't know until I open it up.
The input part of the amp seems fishy anyway, as it doesn't properly load the pickups and brings a lot of noise on the Hi input and a lot less on the Lo input.