Thank you for your detailed reply, I really appreciate it.
I have a good multimeter at home, I'm just not sure exactly how to measure noise floor with it, I will read about it and try it or if you have a second to explain how to do it that'd be great, that's a very good idea!
EDIT: I just had a look at my multimeter and unfortunately I don't think it is sensitive enough, I only have 200ACV or 600ACV.
I am using the XLR input, the source feeding it is a RME ADI-2 DAC FS. The noise is the same with or without anything plugged into it. Should I still try with a shorted RCA?
As for the nature of the noise, it seems to be a 60Hz hum with harmonics, I only hear the low 60hz is I put my head close to the woofer.
What I hear from the listening position is mostly midrange "electrical noise" so it might be 120hz. It's a constant buzz, no crackling, no static or anything like that. Comes from both channels equally.
If I remove the 12AU7 tubes it dissapears completely. V10 and V13 seem to have the most effect. That leads me to believe the noise comes from the driver stage, not from the power tubes.
Thanks again for the very informative post.