I think he was running a single EP600 until Dennis ordered a second sub... then of course Ian had to upgrade as well.
EDIT -- my mistake; it was the other way round. Ian added a second EP600 at home and his results were a factor in Dennis doubling up the 600s in his system :
"Well after having recommendations Alan, and from Amie since Ian added a second, and they seem to have a similar listening area as to size and surfaces. I figured my thought of adding a second was not all that far off. LOL "