i did a lot of experimenting with mine(i got a combo, i like combos)- in the end i stayed with these changes-
R1-was 68k stock, i put 1M
R2-was 68k, i put 10K
these two changes made the biggest difference, as stock was no good
R5- i put a 250k pot here, acts as master volume
R6-1M stock, i removed the resistor and put a jumper across here
R7- i removed the resistor and left open
the R6 and R7 changes allow the amp to distort quicker, with the R5 as master volume, and the R6 and R7 changes, i get a good throaty tone at really low volume-no need for further attenuation. full open it screams
R8 and R9 i changed to 1.5K
R14- changed to 270K
C3 and C4 i changed to 2.2uf 25v- removes the mushy low end
C1 and C2 left stock values but changed to orange drop caps
thats about it- i tried a bunch of different tubes, but the stock ones stayed in
stock speaker is pretty good as is, i left it in too
amp cost me 175$, spent under $20 on caps and resistors- sounds great, makes a killer bass amp too