I saw a post bigjon made about his subs..and if I remember correctly he said he started to damage three subs so he backed off the amp a little...only reason I say they are lower quality is strictly going off rms rating (which may not be a fair judgment) and why wouldn't I have enough room and the ethos only need 3.5 cubes each..I know if I go with a 4th with the sp4's it will have to be a wall