You are right. The maximum power you can draw from a 120 VAC socket giving 15A is 1800 watts.
...
Class-AB with conventional power supply is 50% efficient at most. That gives you only 900W from a 15A circuit. Yeah, it's music, and there's peaks, and capacitors can store power, but 900W continuous is what you're going to get. If you like to hold or drone notes it's continuous wattage you care about.
Class-D with special switching power supplies can get 90%+ efficiency - 1600W+
Class-d is the loudest
Time to call in the FTC, EIA, and EIAJ, or even THX - only believe manufacturers who publish measurements to one of these standards.
You can buy non standard amps and they may work fine for your needs but you never will be sure the power they put out.

