Florian – I finished the circuit portion of the Sunn-T build and tested it with all the stock components. It sounds great!
Then I went back in and tacked a 300K Ω resistor to R1 & R2 in parallel to drop those to the 56K Ω value and tacked a 33K Ω across R4 & R8 to bring them to 560 Ω. After firing it up and giving a careful listen, I was pleased at slight, yet noticeable change. This mod seems to allow the settings to get into the “break-up” zone at somewhat lower settings. I don’t have a scope and could not measure headroom, but I’ve decided I will keep the mod as I like getting into the distortion zone earlier in the Normal & Brite Gain settings.
And, I have no recollection from where I found the suggestion for this, but I’m going to drop a simple buffer between pin #2 of the treble control and pin #3 of the Master volume control. I’m thinking the 100nF cap is not needed, but I’m going to start there anyway. (The buffer circuit is shown in the attached photo.)
If anyone knows from where this mod came,please speak up and let me (us) know. I’ll report back after I test this addition.