4 Velodyne HGS-15's or 2 Velodyne HGS-18's?

If you're using Audyssey (or similar DRC) to smooth FR in the bass, then a pair of 18s probably makes more sense. I believe that you'll have pretty similar clean SPL capability all the way down well below 20hz from either set-up, and 2 giant boys are probably cheaper and easier to place than 4 big boys. As you note, it's probably overkill in either case (unless you got yourself one very big HT room), so there's little chance of going wrong, either way.

If you're not digitally correcting for the room, then 4 subs will probably get you smoother response. In that case, you might want to place the "top" sub on a support that's placed closer to the ceiling rather than stacked on the "bottom" sub. I'd think that this placement would take the ceiling reflections out of play (to a degree) and this should get smoother FR in the bass.

Marty

How about a used 1812? Not easily available, but can really shake the house.

for sure the 1812 not easy to acquire.
I went thru DD-15's X2 then tried Jl-F-113's X4(two for music and two for HT).
No matter using ARO or sub in seat position or any floor position the JL's didn't give me THAT sound.
Enter the 1812,I first heard one at a dealers room which was at least 3X my room size(12X19X7).
The demo disc was Jeff Beck(Live at Ronnie Scott's).
In a nut shell that was the best bottom end I had ever heard.
the dealer gave me a copy of that disc to try at home with the JL's,well I did try it and it sucked big time,talk about depressing.
Went to the dealer and told him it was probably his big room that made the difference,his reply is it doesn't matter the size of the room(HE WAS RIGHT)He brought the 1812 over to my house and we played that disc.
All I could say when the first few notes played was THAT's IT.
The 1812 hasn't left my room as it has the most musical and authoritative bottom end I have heard and she's superb for music and movies.The sub really opens up the mids and highs.
Sold the JL's(YES)and I couldn't be happier.
Never would have thought this sub does it all and made the JL's sound like boom boxes on the bottom end.
Down side of the 1812,heavy,large,price(if you can get one)are around 4999.00,but it's what she does for the music and I'm overwhelmed with joy what this sub does.

Just get one or two SVS PB13 Ultras with an AS-EQ1 and be done for good.....

The OP's question was asked over 10 yeard ago.