The reality of equipment like this (and many other kinds of electronics) is that diversity is HUGE. We run into this when running commercial heating and cooling load calculations. For your sub and amp to draw close to 15A, you would need to have very inefficient speakers, a very active multichannel track (all channels heavily involved at once), and a high volume level setting in a large room. My old power conditioner had an amp meter on the front panel, and the entire system (sub, amps, TV, and other components) typically drew anywhere from 3A to 7A.