I'm using the sub from their 5.1 system as my primary subwoofer, I can attest that it gets loud. Even though the amp is for 5 other speakers as well, I'm just piping the bass from my receiver to it. Bitch pounds like hell and has already gotten me in trouble..
The 2.1 and 4.1 systems have smaller subs than the 5.1, just a thing to note. the 5.1 is a dual driver ported box, I THINK the 2.1 is a sigle driver, so expect 3db less. Not a bad deal at all, but I snagged a logitech 5.1 system for about $110 browsing slickdeals for my mom's TV.