I made some slight mods to mine.
I noticed with headphones there was some background hum. That doesn't cut it for me, so I upgraded the signal wire that goes from the RCA jacks to the input selector with some double shielded coax (2 coax cables twisted with a shield around them) from an s-video cable and routed as far away as possible from the choke. I separated the grounds at the RCA's as the left/right grounds are connected for each input factory. I grounded the outer shield to the chassis on the RCA end and left it open on the switch end. I also upgraded the wires going to the binding posts just because I had some silver plated wire left over from another project.
This made a noticeable difference, but it wasn't totally gone. I wanted to see if it could be better.
It seemed like the hum could be due to the gain of the preamp stage which seems rather high with the NOS Sylvania 6SJ7's I'm using. I noticed that there was some noise that increased with the volume knob, and if I selected the second input which had no source plugged in it was noise city. Too much gain maybe?
So, as a simple attempt to reduce gain, the next thing I did was snip the cathode bypass caps from the preamp stage. I've removed them on some vintage amps I've modded and never missed them.
As expected, this resulted in less gain, and it takes a bit more on the volume knob for the same level, but the hum is gone completely and I'm left with a nice black background with headphones! The amp is now insanely quiet for a single ended amp.
I did a bit of research before snipping these caps and I concluded that bypassed cathode resistors result in a a boost in gain, at the expense of linearity.
The end result of all of this is what sounds like even better resolution, more airiness and transients like cymbals, bells, etc. sound even more lifelike (faster attack?).
It reminds me of a singled ended EL-84 console amp that I triode strapped, but with more control in the bottom end and more balls. It's actually what I would image a considerably more expensive (like 5X+ the price) tube amp to sound like.
BTW, I don't recommend this if you are concerned about warranty coverage. I wouldn't think of doing this on a $1500 amp, but I can gamble with $300.
