Having found time,,,I just simulated the preamp up to C15 tone output,,
A couple of observations.
On Clean the bass response at the output of U1 starts with a hump at 2 hZ which is kinda silly.
But when drive is used the bass rolls off at a more sane 50~60hZ.
I can only assume this would create an obvious imbalance between clean and drive mode.
Maybe too much bass on clean and next to none on drive. In combination with the diodes D1,2&3 would likely be kinda harsh.
The mid control does little to create a mid cut at the classic 400hZ. so yeah proly not nice to play if you want a more fender tone.

I would first remove the diodes and use an OD pedal for dirt and just use the drive mode.
This will give the Amp a lot more clean headroom,, And leave the OD tricks for pedals which do a far better job.
If you need more bottom end up the value of C4.
As for the tone ,, you would have to rebuild it or I as mentioned easier to build an external tone pedal.

Sorry to say but if you try to turn your hamburger into a lobster you may only end up with a bad smell.

Phil.