Only thing I can think of that might be a problem is if one of the subs plugs into another circuit. They must all plug into the same circuit or you risk ground loop hum. Other than that you should be fine, and the more subs the better.
Daisy chain another subwoofer
I have a rythmik f-12 subwoofer that I would like to add another sub to. I'm currently using the line level inputs. Can I use the line level outputs from this subwoofer to connect another subwoofer off this one? Thanks!