Setting up multiple Subs can still be worthwhile, but it has been argued by many that if you are only worried about a single seat then you can forgo them for one well placed Sub.
That said, I have a small room which I use for my personal listening room and theater... one seat... 2 Subs.
To me it's worthwhile.
Fortunately, your room is not truly square. Where an inch can make a significant change in acoustic performance, you've got a whole foot to play with by your measurements.
Probably the easiest way to hear how bass will perform in your room is to use the Subwoofer Crawl. It is crude, but effective.
Do the Subwoofer Crawl. It's the easiest way to find the proper placement location for your subwoofer to get the best performance and sound.
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Also, if your room is rectangular and sealed from the rest of your home, you can fall back on the Welti-Devantier approach to using symmetrical placements like opposing corners, 1/4-wall, 1/2-wall, etc. The key thing about this latter approach is that it is predicated on pairs of subs added.
My understanding of Geddes, on the other hand, is that he encourages removing Subs from the front wall, especially if your Mains are capable of full range, and placing them one at a time in the best measuring spots around the room.
Perhaps I got lucky in my room, using a strategy based on Geddes and the subwoofer crawl combined. I have not sat down to measure everything as I should to prove my setup, but the "before" graph from my Audyssey App is pretty good.
More importantly, and easily overlooked is also how you set up and place the rest of your speakers and LP. It is very important to keep your mains off the front wall by at least a foot (and I also prefer to keep mine out of the corners by at least 2.5'), toe in, etc. Likewise, Center placement should be strongly respected and is just as important as the L/R. Keeping the Baffle of the Center at least 1 foot away from the screen, aiming it towards your shoulders (I never like being directly on axis, but within 15º is usually very good)...
In the end, when your room is well adjusted and you are getting good measurements from the speakers, adding the subs one at a time and taking measurements to confirm their placement as you go would be the Geddes approach. Or place a pair or 2 of Subs symmetrically in your room a la Welti-Devantier.
Lastly, considering your question here, are you set up with REW and a Umik1 to do your own measurements?
