While I understand a *general* rule of thumb is the larger the room the more likely a vented sub(at least from power Sound) may be the better choice to a lot of little details that can sway this pick one way or another. The room layout, intended placement of the subs, listening habits with both music and movies, seating position(s), distance between subs and the seating, the type of subs in use now, etc.
The size of the room only really matters in terms of pressure vessel effects---what we can expect for room gain. To predict headroom(clean output at the seating) it's largely irrelevant although we do need to consider the pressure vessel effect to some degree in that context.
If we're looking at say one sub in/near a corner of the room and one sub against one wall...and an avg of 12ft from the seating(this is a generic example---not sure how it applies here) dual S1800 subs should provide 109-112dB at the seats in the 16-25hz range. That's pretty loud for the 99% out there. And the dual S1800s would have the edge in the mid and upper bass. So with music, win for the dual S1800s for sure. With movies, arguments can be made either way.
If your listening session is limited to a few of the nuttiest <25hz scenes (which tend to be in the *worst* movies I don't know how you guys even sit through some of them.
)...AND you're pushing the master volume levels above say -06 to -09(under reference) the dual V1500s will begin to offer more of the tactile sensation associated with those scenes+levels. The closer you push the system to full reference the more audible the advantage for the V1500s will be.
HOWEVER, settle in and watch an entire movie(again, at say -9dB under reference) and I would bet the dual S1800s would compare favorably to dual V1500s.
One more comment. I saw someone worried about the integration to the two existing subs. While that is always something to consider in this case the old "add ported to ported" rule of thumb doesn't really apply. We can just ignore the little infinity altogether. It's going to do more harm than good. The vmps is difficult to predict as its design is unconventional (one 12" active, one 15" active, one 15" pr
) but given the limited knowledge I have on it---I'd guess that the dual S1800s OR V1500s would sound best without it.
Power Sound Audio