My space is larger than
, so my single Cap 4000 has a lot more volume to fill. That said, a single Cap 4000 has no issue pressurizing my space and overwhelming me at the listening position if I choose to push the volume.
After hanging out for a few hours, Brian and I quickly realized that our listening habits are pretty similar; we are not 135dB guys by any means. While demoing John Wick 2, he briefly pushed the volume a bit higher for a demo scene (I think the peaks were 108dB) and we were both thinking...yeah let's turn it down a smidge.
My point is that a single Cap 4000 is probably more than enough for most people, in most rooms- even big open spaces. It's worth noting that my room is over wood framing (crawl space) so maybe if you're on concrete slab it would make a difference.
The biggest advantage to 2 subs isn't so much for max volume levels, it's really to smooth out frequency response at the listening positions, so 2 subs is certainly desirable if you have the space and commit to best placement.
Unfortunately for my wallet, I'm a HUGE fan of symmetry
, so I definitely would consider adding a second Cap 4000, but it wouldn't be because 1 is lacking in any way.