One obvious thing to do is to take the the E15HP to your house and hook it up where your F12 is.
It should equal or better your F12. If it doesn't, defective sub is a safe bet and comparing it to the second E15HP is suggested.
If it sounds fine at your house, then you know it is his room and placement (or the AVR) that is making it sound poor.
While you are at it, maybe he can let you play with it and your f12 (as dual subs) so you can get a sense of what benefit a second sub would get you (sometimes it is major, others, not so much).