FYI, I have since come across one tone stack tweak, on 18watt.com of all places. OK, it was in the context of a thread about the Marwatt.
rjgtr wrote:
On the Marwatt schematic, try replacing C14 (the .001) in the tonestack to .022 or even .047. That will increase the bass, but the amp will still sound like a Hiwatt.
rjgtr wrote:
...The DR is definitely more of a guitar oriented design. But the CP is what Pete and John used. Interesting, both also have a little more low end than the DR. You can simulate the same effect with the DR stack by changing the .001 cap between the 100k and treble control to a .047. I did this on my DR103 and is a lot fatter sounding.
I think that one's C6 on the Triwatt schematic. I guess you could use various bigger cap sizes like 5nF, 0.01uF, 0.022uF or 0.047uF, depending on how much you want to fatten the tone.