If you use a pot for R6 then the gain for low frequencies will be changed differently to the gain for high frequencies, unless the capacitor to ground is huge which will take time to charge when the circuit is first turned on.
But a pot for R11 will reduce the gain for all frequencies equally.
I thought your oscillator will sweep frequencies or jump between two frequencies? You can test the sound levels by feeding the amps with a continuous oscillator always at the same single frequency.
I drew my circuit wrong and you copied the error. The emitter of the transistor, D1 and R9 should connect directly to the (+) of the battery, not to C5. Then your 'scope waveform will probably be squarer.