With the gain on the amp set wherever my installer set it, my empeg set to -20dB is decent but a bit quiet. My Alpine set to 18 (of 30) is comfortably loud. At 0dB, I can tolerate the volume of the empeg. The Alpine, let's just say I've never gone to volume=30 for fear of damaging my (stock) speakers and my ears.

I wish you'd provided like for like values. 'decent but quiet' != 'comfortably loud' != tolerable.Maybe the Alpine has 8V pre outs vs the 4V outputs of the empeg. Assuming that is the case, with a flat EQ/Bass/Treble on both units, I'd expect the Alpine at 15 to be roughly the same volume as the empeg at 0dB. (Obviously, check on the same song - use a CD that you've already ripped and put on your empeg.) That 18/30 Alpine volume equates to 4.8V, or about +1.5dB in empeg terms. 30/30 would be +6dB in empeg terms. You might want to uncap the empeg's volume and confirm this. (Can't remember offhand where that setting is... config.ini?)

Buy an attenuator for the Alpine and boost the gain at the amp.