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

one of those 1 into 2 splitters would appear to be the way to go. ONLY proviso would be if the impedance drops too low. This would only happen if both subs are fairly low to begin with.

If subs are located near the amp, you could also go with speaker level. Are speakers full range? Speaker level input on a sub is very hi impedance so it won't load down amp.

I've seen some plate amps w/both low level inputs and outputs.

the Human Ear is fairly insensitive to Bass Distortion. Several % should be doable, if not more. I know there is research, but I'm scared to look it up. That same amount of distorion at 5 or 6khz would drive bats from the room.

EQ'ing a room may make a certain seating position a sweet spot, but Will Not effect room modes. They are a function of the geometry of the room and sub placement. Multiple subs will excite the room and produce, best case, complimentary peaks / nulls which will give reasonably flat response over a larger area. Also, I suspect slightly lower distortion at higher levels.

As for X-over frequency, I cross my Maggie 1.6s at about 40 to 45hz. Lower limit for the Maggies is about 40 or so? Maybe 36, depending on who you read. Anyway, I dial 'em in for a little overlap, so as the maggies drop away, The sub picks it up. I do NOT use test tones / meter since I'm nuts anyway and this would be just like trying to put out a fire with Gasoline. Like, how obsessive can I get?
Besides, there is an art to the science of SPL measurement and you'd probably also need someone to help you take the data. Multiple repeats, a spreadsheet and I've finally flipped. Sheesh, Science ain't easy!
I may be able to wring a little more performance out of my system if I were to hi-pass the Maggies at 60-70hz and the lo-pass the sub at maybe 50-60hz. I will probably NEVER find out, unless a few volunteers show up with a bunch of stuff and some Single Malt!

Also, from our 'for what it's worth' department, Somewhere above 80hz or so, you can begin to localize the sound, so you gotta be careful there, too. With my current setup, I simply can't find my sub.

Thanks so much for the kind offer! Even though I am on the 'left' coast, feel free to drop in.
But, I must respectfully differ. One theme running thru many posts is that measurements DoNot matter. The fuse guys and some of the cable guys are good examples. Nobody has demonstrated a way to measure a fuse good/bad. So, too, the cable goes unmeasured except for the measures of resistance and Capacitance we are all familiar with...and yet! there is something more to it.
I can see myself (I know me!) going absolutely nuts with an SPL meter, test tones or even analysis software. Since in my professional life, I am a cleanroom technician for a semiconductor manufacturer, I am Very Familiar with experimental design, and the use of JUMP analysis software.
Nope, I'm going to leave it fun, just listen and treat those known defects of my room. My room, just for laughs, has 8 sides, is very unsymetrical and has an off center 12foot vaulted ceiling.
Now, if I had a certified / calibrated microphone..........

I may do so, in the future. BUT, I know my character and once I start down this path, I'll be nuts until I figure it out.
Yes, there are 2 schools of thought on this 'hobby / avocation'
1. Just listen and enjoy
2. Analyze and measure
I DO have a radioshackup SPL meter in Analogue form AND I sent away for the Rives Audio disc which has test tones calibrated to the RSmeter. The RS meter has known rolloff curve so, the tones are biased.....either that, or they provide a 'if this....add or subtract this much.......table.
I don't know! I never opened the box! I figured, 'There Be Madness'.