Here is how to modthe input pregain.
to add the pre-gain control from the ggg octavia you just need to put a 50k linear pot between the input and the 100n input cap. On the pot - 2 goes to the input on the board, 3 goes to input on the bypass switch and 1 goes to ground
Here is how to mod the octave off.
http://fuzzcentral.ssguitar.com/octavia.php
build and audio samples coming up.