Even if you will build both channels I think the input stages in each channel could be used.
You eliminates some channel switches and you can use them as two separate preamps for 
instance. 
                                                                                              

The signal passes through the input stage with some frequency boost. If you overdrives stage 
one (turn up GAIN 1) you get more course distortion, and if you do the same thing with stage 2
(GAIN 2) you get a finer and more compressed sound, on condition that the LED's are activated. 
Otherwise you get sound close to stage 1's. The difference is the OP-amps (TL 072 and LF 351), 
there LF 351 has a softer sound.

The low pass filter cuts the highest overtones.
                                                                                            

You can overdrive the EQ OP-amp as well; Turn up the GAIN 2 control and switch off the diodes.
An important thing is that I have listened through a Celestion 12" speaker that has its 
special sound. Therefore the component values in the EQ might be modifyed, especially the 
middle control.
                                                                                            

If you build both channels you can lead the signal from the master control to a mixer and 
perhaps to a reverb driver.
                                                                                            

The voltage should be about +-15V.