Another Schroeder user here (not the same cab -1515L - but there's a family tendency), and I've never had volume issues with any of the heads I've used, which include a LMIII, RH450, Ashdown Spyder, Streamliner 900, SVT7, and a Mesa M-Pulse 360. Can't help but think there's either some thing wrong with your rig, or you're unbelievably loud. Do you use ear protection? You really should.
When we set up, we do it a bit like studio practice: once the drummer's good to go, he and I play through a few bars to get our levels right and possibly EQ for the room, then the guitars and keys and then vocals set their levels to that WITHOUT having us turn up. It prevents the guitards setting the bar far too high (loud) and keeps the volume wars under control. As has been said before, make sure the guitards have their cabs pointing at their heads, that normally keeps them happy without everyone else suffering.