You can insert a pair of Rothwell attenuators between the pre-amp and amp. Technically this does not reduce the gain of the pre-amp, but it does attenuate the signal that is input into the amp. Effectively it is the same result. It will require the volume control of the pre-amp to be set to a higher level to generate the same output volume at the speaker.
Enjoy,
TIC
Enjoy,
TIC