The problem is in the distorted channel you already are at max preamp volume and gain, so there is no where for it to go. You are basically maxed out.You would need something in a send and return jack of the amp to change the gain to the power section.

Well there are many ways I can think this could be done and not having your amp in front of me to try is hard.
Does your amp have an effects send and return jacks on the back?

Are you looking to have your distorted sound the same but just louder when you press a pedal?
or are you looking for a slight crunch and then full distortion when you hit the pedal?

Just as an experiment try the LPB-1 in the loop and see if it works… not sure but worth a shot.
Other wise a volume pedal in the loop or a “signal pad pedal” may also work in the loop. Not sure of the impedance issues using the loop but it could work.

You might want to think of it backwards! I know it’s counterintuitive, buy you might actually want to use an attenuator to CUT the volume and then turn OFF the attenuation when you want more volume. That way you’re not introducing more gain into an already high gain sound. That is one of the use cases for the EHX Signal Pad:

https://www.ehx.com/products/signal-pad

They demonstrate it being used that way. So, you would first set your high volume for your solos, then you would engage the signal pad and cut the volume down to where you want it for rhythm. You would leave it engaged normally and disengage it to go full volume for your solos.

You could experiment with placing it at different points on your effects chain including your effects loop since your amp just sees the attenuator as an effect.

If you put it in front of your distortion then engaging the Signal Pad will decrease the gain going into the distortion. If you put it after your distortion, then it will only affect the volume of the distortion, not the gain.