TRL Dude or Joule 150 MKII for Major Pre Upgrade

grid stop resistor on the 6SN7 of the Aesthetix
OR less likely...
RCA connector on the Aesthetix shorted audio ground to chassis.

Grannyring,

I have no experience with the Bel Canto but when your passive lightspeed is set to vol = 0 can you measure zero ohms at the output RCA relative to ground, much like the shorting RCA jack you tested? If it's zero ohms then I see no reason why connecting to the lightspeed would be different than shorting the inputs with a shorting RCA.

Recently I spent many hours tackling an interesting project on a previously modified Jolida which gives a buzz similar to what you are hearing. Turns out that power amp uses an AC filament for the gain stage and the center tap of the power transformer is faulty and I had to create an artificial center tap w balancing resistors.

Highly unlikely with your hybrid power amp cos judging from the build ($$$) it would likely use DC filament for the 6SN7s.

Check the input grid stop resistors again, it might be as simple as adding a 1k resistor inline with the signal input to the 6SN7s grid.

All depends on how long that curcuit trace is from the RCA hot pin to pin1.
If its less than 3 inches then a 1k grid stop in series ie
UNSOLDER RCA hot and put 1k inline.
SOLDER Socket-hot-->1k-->wire to grid@6sn7.
But if its any longer then you will need help from Aesthetix.

Acck! I now have to say hold everything cos I just looked at my NP220PG and the grid stops to the 6SN7 (also in diff amp config) is 100 Ohms. They are place after the grid leak which is 100k to ground.
So if my NP100PG is so darn quiet and EM free even with crappy cables whats up with Grannyring's Aesthetix?
My NP has very low gain though, my speakers are 89-90dB efficiency.