There's no voltage gain on a transistor emitter follower, so that leaves the opamp for gain. The gain of an inverting opamp stage is 1 + (Rfeedback/Rinput) (I think). That means the gain is set by the complex resistance networks in the tone stack and varies by frequency. That would take some working out to determine what the gain is, but I think you can adjust it by sticking a pot in the feedback line from the opamp output to the tonestack. Use lugs 1&2 to set up a variable resistor and see what happens. Start with a 100k pot.
Disclaimer: I have not tried this before, so this is purely theoretical advice. Worth a try though, I think.