OK, my suggestion is change the whole volume control stage. The original design is a compromise, in my opinion. First remove the .002uF cap. The 'Lo Boost' isn't really needed now and it can be jumpered (ie the 10M resistor is jumpered out). Replace the .0075uF cap (or .0068uF in your case) with a .1uF coupling cap (450V would probably be fine). Then yank the 500k pot for a 100k audio taper pot. So that stage will just be a .1uF cap with a 100k pot to ground with the wiper to the next stage. You should be able to get these parts at a local radio shack for a few bucks. For even better performance, I would get an even bigger cap, like a .22uF or bigger, though .22uF is plenty. I would also drop that 68k grid stopper at the front of the unit in half - anywhere from 20k to 40k - whatever you have on hand. A 1/4W value would be fine. We want to move the first Miller Effect pole beyond the pole created in the second stage which has now been changed.
The results are you have a little less gain in this stage, but not by much. Basically, we whacked the high frequencies down in line with the bass. The frequency is very flat, totally determined by the cap and resistor for the low frequency and the in series resistance (hence the smaller value volume pot) and Miller effect of the next stage (a -3dB of about 50kHz). We haven't killed the Sunn design at all. The volume control was just a weak point, probably due to the expense of the very large coupling capacitors. Now you have a good flat EQ going into the tone controls. You should be able to evaluate your amp better now.
Shannon