I added a REL Q108II as I needed something I could put in a window out to my deck from time to time to add bass to a separate amp/speaker rig (signal from the same preamp however)...when not in the window the deck amp is off, and the 108 remains in the main rig getting its signal from the main power amp. In any case, that's how I wound up with 2 subs and somehow 2 really does seriously add a lot to the overall musical fun.