My Lakland with Nordies and an OBP-3 is hotter. I understand what you are saying, but I didn't think volume related to other basses was the complaint. I thought the bass was so hot it would distort. I do not have this issues running into my RH750 or my Shuttle. And yes, I can turn mine all the way up. The volume knob isn't great; there isn't a big difference between one and ten
I will say that I find the electronics to be a little noisy - not intolerably so and not unexpected for a $300 bass. I also only run the bass flat. The treble is a little harsh and the bass sounds artificial. Fortunately, the flat eq sounds good on stage, and FOH is whatever the soundman makes it anyway.
Edit: regardless,I like your plan. The basses are an electronics revamp and an ugly bridge away from being a much more expensive bass. Which EMG are you using? How much will the pickguard have to be modified?