It's a speaker/monitor module that produces low freq sounds. Some people love them, some despise them, so give listens to a few setups before deciding. I have a Polk PSW10(? I believe, not fancy at all). I use it for referencing with my speakers/monitors. I will say, with PA's you won't really need a sub. I have PAs too, but prefer the sub because I can turn it on and off when referencing. PAs are always LOUD(but great when company is over). I did used to prefer them when I blasted my music at all times, but these days I'm doing mixes while my kids are sleep. PAs don't work on any setting except "turn house into club", but if you can blast music, I mean to the point where a neighbor is not a factor because there's space between homes and you have no sleeping kids, I'd recommend PAs all day.