#1 Am I to assume that the subs you listed will blow the Def Tech SC4000 away?
DefTech is known to be both vague and "optimistic" with their specs. The SC4000 is no exception:
- 1,200W ??? (RMS? peak? something else?) amp
- extension to 16Hz @ -??dB (-6? -10? more?)
The subs listed by shadyJ
should have no trouble outperforming the SC4000.
#2 HSU has the Hsu VTF3 on sale for $649, so that is perfect. But is it overkill for the speakers I am purchasing?
No. Once your system is calibrated, the sub should blend with it.
... Guttenberg loves it but says it is overkill for rooms under 500 sq. feet ... Although, the back side of my HT room is open to another 20 x 20 room with a bar and poker table.
So, you've got a 400 sq.ft. room that is open to another 400 sq.ft. room. 800 sq.ft. x ~7.5' = ~6000 cu.ft. That is a HUGE space. A small sub will be overwhelmed by it. Even a single better sub will be put to work pressurizing that much space.
#3 I have never considered it, but I can fit two subs easily. What are the benefits of having two with an application like mine?
Greater overall output, increased headroom and smoother frequency response across the listening position.
As for how to proceed, IMO you should buy one good sub now, and add a second good sub later on.