Inside Rx electronics dualtone pedal - mods??

[dubs]

Hi I have a prescription electronics dualtone pedal. A cool pedal, but is quite fuzzy at high gain and no sustain a higher gain levels, just signal kinda  dies off after a few seconds. The pedal is not faulty, just characterstic of this pedal.
I changed the original TL071 opamp for a opa134 which seemed to smooth it out a fair bit at high gain (higher slew rate)

Any suggestions to mod it to:
- make it more transparent ( quite warm in tone)
- sustain longer?
I'll try to do schem/layout, but here's some pics: