There are a couple of different ways to do this. If you're using a preamp that has a sub output (as most home theater versions do), you can adjust the sub output with the preamp. In my case, I bought an amp that is adjustable (a QSC 1304). This amp allows you to adjust the level. In addition, I'm running an active crossover, which has additional controls for adjusting level, phase, crossover frequency, etc. I run my RM40s full range and then run the entire range to the active crossover and from there run the selected low frequencies to the amp (which I turn all the way up, or to a high level anyway.