Andrew- With mono amps, it is always a possibility that you have a slight ground loop problem, particularly if you already know that there is a grounding issue with the house. Also, other devices that are on that A/C circuit can easily manifest noise onto the line that feeds only that amp (assuming you're using different outlets/circuits to power the two mono amps). Lights, refrigerator, fans, TV, computer, etc... can all be culprits.
If you think it's the pre-amp, simply swap the output cables at the back. If the noise appears in the other channel, then it 'could be' simply a tube issue.
Try not to sweat the small stuff, and enjoy the music.
Just my .02
If you think it's the pre-amp, simply swap the output cables at the back. If the noise appears in the other channel, then it 'could be' simply a tube issue.
Try not to sweat the small stuff, and enjoy the music.
Just my .02