My modded combo ValveState40 (8040) reverb tank has started a weird buzz on the low A on the 6th string is played - just when the reverb is enabled. I figured since reverb tanks are pretty cheap I'd install an external replacement on the floor of the cab. But I can't figure out what the input/output impedance would be. It's not on the unit (which is mounted on the mainboard) and despite much Googling, I can't find any methods on how to measure it with a multimeter. Does anyone have any idea on what the impedance would be for the reverb tank? It's quite small.