Redrob,

Audio is logarithmic. Every +6dB is twice as loud, and every -6dB is 1/2 as loud to the ear.

If the display is showing actual dB changes, then lets say:

System A: 0 = 75dB
System B: -6 = 75dB

When we turn System A to -6 and System B to -12, each has gone down 6 dB, and thus is half as loud as before. Also since we know the calibration amount, if the display is dB accurate than we know they both will be at 69dB.

Also with your sub amp. 2 @ 4 ohm drivers in parallel are a 2 ohm load. When bridged, the amp sees half the load, so you are giving the amp an effective 1 ohm load. Good thing it is a Crown, few other amps could survive that.