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

I wouldn't stack the 15's and I would suggest putting the subs outside the main speakers. Almost no manufacturer reccommends the inside the main setup. If you want four subs put them in four corners. The Velodyne 18's are great so use em. I personally have two Revel sub-15s and they fantastic both for music and HT. In the scene in Matrix where the helicoter hits the building these things are bone jarring.

Get the 18's and put them in corners (or close to.. if you cant tame them) and make sure your homeowners ins covers
structual damage from non-natural sources.

Get a Earthquake Supernova MK-IV 15 or a pair and get ready to be stunned.

I had a Velodyne HGS-18 and thought it was the best sub for a HT/music combo. I happened across a deal on a Earthquake Supernova MK-IV 12 at a divorce sale for only $400 so I picked it up.

WOW!!!!! It was so much better than my Velodyne that I placed the HGS-18 up for sale a week later.

The Supernova 12 is just stunning in it's shear power that it makes some movies scary. It is not as good as a REL for music but just as good as the Velo.

I finally feel vindicated by the glowing review from Stereophile GTHT. No one ever believed me when I said that this sub is better than the HGS-18 until they heard it themselves.

The Supernova MKIV 15 is better than the 12 I have but I don't think my house could take anymore pounding.

I do like the idea of using a pair of subs since the room I have responds much better. I used to use 2 subs for left and right and the Velo HGS-18 for the center/surrounds/LFE but since adding the Earthquake I only use it and a old Kinergetics research sub which is hidden under the coffee table in the center of the room.

The Earthquake Supernova subs are the real deal and you would be making a mistake if you don't compare it to the HGS-18. I am sure you'll be a convert too and save a lot of money to boot.

A lot whould depend on the volume of the room/space being driven. However, It's my experience that you're NOT LIKELY to have "too much woofer", as long as you correctly match volume(s) with the main speakers.
For the record, most people have the subwoofers overpowering the main speaker(or mid/treble). But that said, often, there is not enough sub in SOME systems, or larger rooms, to properly handle the demanding bass dubties!
However, a lot depends on what volume levels you listen too. So for instance, it's possible you never push the volume level up much at all. Thus, some people can get away with a single, or marginal subwoofer situation.
In your case however, this will not likely be your concern.
It's been my expience(and I've had a lot), that you can potentially smooth out your bass in the room with a couple of woofers next to eachother in a room, and thus smoothing out some bass modes. So, 2 stacked, or 2 side-by-side. Each room is different, and results will varry.
If it were me, I would usually go with 2 stacked 15's on each side(4 total), over 2 18's. Experimentation and strong consideration of all the location options is recommended.
I would however avoid the often recommended "corner placement", as that ALWAYS LEADS to a "less than accurate" bass pressentation! YOur sound will be boomy, and "one-notey" in the bass, and you'll get "bombastic" sound even with instruments covering those frequencies covered by the woofers!...NOT NATURAL, OR WHAT WAS RECORDED!!!
The only time I'd place woofers in the corner myself, is if the sub was underpowered for the room, and needed some bass reinforcment that a corner placment would give. Even then, I'd be doing some SERIOUS EQ'ING of the woofer! So it is workable, but not as ideal. And in your case, you'll probably have more than enough woofers I suspect, so it shouldn't be an issue for corner placment.
Someone else in these posts also mentioned the Earthquake woofers. I must concur that the Earthquakes make stronger HT woofers with more "bight" and "weight" to the sound!
The Velo's are good in my oppinion for HT, but if I had to choose, I'd go with the Earthquakes in that regard I think....IMO.
Good Luck

4 velos.