Neither him or I know anything about building one from scratch
He already knows more about it than I did when I made my sub box

... I bet he even has tools!
The #1 thing about subwoofer boxes is bracing. I would recommend a cross-brace at every roughly 8-10 inches.
some really high quality walnut HDF he wants to put a gloss finish on. 8 cu feet. for 15" LMS-R 15?
I hope you have a forklift




Any tips on what kind of driver, porting, and volume I should go with if I do one in the front left corner, and another on the left, and a little ways behind the sitting area?
Come on down to the AH DIY forum and we'll model different drivers, enclosure sizes, and enclosure types for you. Some guys have better programs they actually paid for, but I can run you through some winISD models myself.
It depends on the driver's sag factor, and there's a program that can calculate that quickly for you. Personally I'm just not comfortable with downfiring. The idea's always at the back of my head that eventually the spider is going to get damaged after a long time. But people not only do it but have great success with it. I'm just paranoid.
I watched a youtube video of that LMS-5400, actually wrinkling the body panels of the SUV it was in the back of... I'm looking for some good room pressurization/impact and something that can go low enough for any movie soundtrack... but not something that's just rediculously out of balance with the rest of my system.
The first key to balance is calibration. If the sub is calibrated to the same level as your mains, it'll never be an issue. Where something like an LMS-5400 gets useful, is on the loudest transient peaks in music as well as the dynamic range of subwoofer content in movie LFE which is up to 10db louder. If your processor is calibrated well, you'll have a hack of an experience because essentially all of the drivers I recommend earlier are very linear and low distortion.
My main reason for wanting to go with two isn't so much for an increase an output, but for even room coverage.
Excellent. The other key to balance is to deal with the room. I would even go with four lesser 15" subs (like the Exodus Tempest X2 perhaps?) if room placement allows it over two superior subs. Seal those up and you would not only get great in-room sound but also deep deep extension.
I want to avoid like the plague, a one tone droning subwoofer as I'd like to use something of a quality to compliment but not offend the rest of the system.
Good
DIYCable.com is that one sub amp he shows there junk? Never heard of it before.
Kevin Haskins does not sell junk. Everything on his website is excellent, although some of his cable parts are on the overpriced side compared to a monoprice or something. With that said I don't know if 1000w is enough for some of the lower sensitivity drivers we're using. For some it is, for others you end up hitting the amplifier limits before hitting the driver limits... which is fine, I guess, but kind of irritating.
What a lot of us do is run the subs "passive" off of a pro-amp. AH actually just reviewed a funkywaves sub the other day:
http://www.audioholics.com/reviews/speakers/subwoofers/fw-12.x
It's passively run off of a high power pro-amp just like we recommend. THat however is the 12" LMS-R, and us, well we're a bit greedier
I'm also still looking for some more suggestions on reasonably priced, reliable, high quality amplification
Honestly, Parasound, Emotiva, Outlaw and ATI B-Stock are all rather great choices. I would also consider the Face Audio amp I recommended earlier but that gets into pro-amp territory and the biggest drawback with them is that at lowest listening levels you might run into some fan noise if the amp is in the same room. The big advantage of the pro amps is that they're more than often 2 ohm stable so you can run virtually any speaker off of them. That and an abundance of watts. THe Face in particular is very low-noise... it's almost designed like a high end audiophile amp . And yes, ADTG, i think those aesthetics are gorgeous. I'm a sucker for a good champagne finish. One day i'm going to waste like 8k on a marantz sa-11s2.
If the amp is in a closet you're probably home free. Another option is quiet-fan mods, which a lot of us are guilty of as home amps don't need fans the way constantly max-power pro amps do.
http://www.avsforum.com/avs-vb/showthread.php?t=976373
Of course I still lean the way of home amps myself. But if I ever get a "dedicated" HT room, that face is at the top of my list.