Well, if you calibrate two different systems to the same reference level (so that they both produce a 75dB test tone when the volume is set to 0, for example), then even if one uses super-efficient horns and another uses very inefficient speakers you should get the same SPL levels when the volume controls are in the same position. The only difference would be how much power one or the other is using.

Now if you calibrate the systems differently, then obviously comparisons are going to be difficult or impossible. With the combination of speakers and amps that you have (and with those speakers I can't imagine the Crowns ever getting even a little warm), it would be easy to calibrate so that -20dB is reference - in which case you could take other people's preferred volume settings and subtract 20 to get a reasonably comparable volume in your system.