not sure but i thought the super sidekick needed an extra wire, or contact or something to work with the 2000gtl.
might try a stock mic and see if it sounds clear.
if you, or anyone else has clipped, removed, or added resistance to TR24, then change it back to a stock 2SC945 transistor. this is most likely the case, as messing with TR24 also messes up the SSB ALC circuit, and not in a good way)
put TR24 back to stock.
if you must clip something, clip R131, it is only for AM and wont affect SSB .
VR12 is the modulation pot in the 2000GTL. VR7 is the control for the modulation meter. (this is different than 148gtl/grant xl, etc...) VR11 is your SSB power control.
set it for a max of 15-16 watts PEP.
usually a muffled sound is caused by overmodulation, so you might try running the mic gain at 12 o clock if the radio is modded for more audio.
of course, you might just have a low mic battery.
hope this helps,
LC