It's not actually that much louder but it's fuller. It used to be middy and trebly and leave lots of room for me. It's still not super deep or loud (he's still got an 18-watt head), but there's not as much room for me as there used to be.
Sounds like the guitarist actually has a 36-watt version. Tell him to just switch out a driver or both in his 2x12, and fit his volume into the mix. If not fire him. If you cannot, then leave. Your hearing is far more precious than the band.
. Heavy but with the wheels it was one trip (head on top of cab) from car to bar, and 2 trips from bar to stage.