A huge output? Like the signal coming out is too hot?
The schematic shows a master volume, are you saying it's not effective to just turn it down there?
Keep in mind, the AB763 preamp was designed to drive a power amp to full volume, which needs on the order of 5-10V peak signal in the 20W models. This is WAY too much signal for a pedal, which even if driving a line-level input only needs around 1V peak.
IMHO, the best way to cut gain in these common-cathode tube stages is to split the plate load resistor (R8, R4). For example, if you split R8 into two, 50K resistors, and then tap the signal from the middle you would theoretically reduce the output by half, without effecting the voicing of that stage.
If you want to experiment, you can substitute in a 100K pot, and take the output from the wiper, and adjust it until you get the maximum expected signal you need with the master volume at full. Then, you can measure it and substitute in two fixed resistors. Be forewarned: the pot will have DC across it, and will scratch LIKE HELL, but for testing it's ok.