There's a number of ways to go about getting all of your individual rigs volume matched. I've found this one to work really, really well. Use a combo of your DAW peak level meter along with a VU meter for RMS. This is the perfect solution for me to balance out a sparkling clean rig right next to a raging compressed (and scooped) Boogie one. The peak meter is run stereo while the VU is mono. Keeping an eye on both (I start out with speakers muted and then fine tune with my ear) is, if nothing else, interesting.
Edit: the VU reacts most realistic it set for a 10ms delay. This works great for matching PUs too.