In the last half hour my Joyo Bantamp (50W hybrid) has started making farty/crackly noises if I dig in on the E string. I have volume low and gain modest. It seems to be very biased towards low frequencies. It doesn't sound like distortion as much as a really noisy volume control and appears to be independent of gain and master volume settings. I can turn up loud and play gently without the nasty noise. Turn it really quiet and dig in, and it crackles and snaps.
It's a hybrid amp, and my worry is that something 'mechanical' has gone awry in the 12ax7preamp stage. Is this possible?