Subwoofer: room 16 x 18.5....

While floor area is important, the height of ceiling also plays a part. Room size in ft3.
Go to the HSU Research site and look at there room size chart.
That'll give you some idea of sub to room relations.
In any event, 8" is too small a sub, except possibly at low levels?

Measurements of subs, while a good thing doesn't cut it in a real room.
Google:: 'room mode calculator' and look for nodes /peaks and resonant frequencies below 300hz. they're the worst.
Also, Find the Harman White paper on subs. The bottom line recommendation is that 2 is MUCH better than 1.
In MY opinion, for a budget of over some amount, for me, about 1500$ or 1600$, it is actually better to have 2 subs in an asymetrical setup to reduce room nodes, both plus and minus. A pair of 800$ HSU research VTF3's would almost certainly be a match for ANY equivalent value of another single sub.
In my case, with a completely wacko, room of no less than 8 sides and an off-center vault ceiling, I make do with a lesser single unit. I'm not a crawl-around-the-room with an SPL meter playing test tones kind of guy, but I can detect no boom or suckout....except in my den where there seems to be a midbass peak which exagerates bass drum. I guess my den my be acting as a helmholtz resonator.

Seriously, check out my Google recommendtaions and have a fun read....shouldn't take more than an hour git-2-go.

Just looked up PEQ. The recommendation to put the sub where you sit than eq away will have you end up with a single spot...where you placed the sub, as the 'premium' seat.
If you go to the trouble...and man, what a hassle, of putting the sub in 3 or 4 spots, running the eq routine, recording the results THAN entering an average value at each frequency, you may get somewhere. But, overall, you've still got the same room you started with.
My suggestion was to take your budget....and I'll admit it has to be above a minimal value, and divide it between 2 lesser subs. In this way you can minimize room interactions.
The Harman White paper is clear on this point.
Also, for Pete's Sake! the Velodyne is 5000$ (gasp!) For that kind of loot you could get 3 or 4 top-line HSU's and associated cabling. HSU recommends a near-field sub.. maybe just behind your sofa or as an end table. The other pair could be located in other more discrete locations in the room.
I would maintain that a pair of good subs totaling 1/3 the Velodynes cost could easily equal the performance and output of the 'Dyne and have less complexity and therefore less to go wrong.

I think you misunderstood my measurements of subs line.
Measurements of subs, like frequency response and max spl are near-meaningless.
Measuring for location is more advantageous, but still a hassle. Room boundry situations make such measurements in the hands of most of us, me included, kind of dicey.

I would agree that 'Spending more money on a sub with measurements comparable to a less expensive one has zero advantage' So, for the person who is going to spend/budget say.....1500$ for a good sub, what would be wrong with a pair of lesser units for the same money?
Link to article:
http://www.harman.com/wp/pdf/multsubs.pdf