Its isolated to the resonance control and that tube position. Not a tube issue, tried many new ones. The sound is very very faint and you may not even notice it till your head is near the grill cloth. All controls set to zero it is extremely faint.. turn up the resonance and you can hear a slight crackling becoming slightly louder. The amp works an plays fine. The crackling is not causing any on stage noise issues because it is very faint. Just curious if this is inherent by design. Can you JVM owners try and see if this is normal on your amps? Thanks!