Subwoofers and Seating Position for Klipsch Forte iv's

You might look into Distributed Bass Array or Swarm setups.

What about reconfiguring the room to have the speakers on the long wall instead?

From your options I’d go with two Rhythmik F12Gs.  For placement and to excite room modes the least put one of the subs close to the walls at the midpoint of each of the front and back walls or the two side walls.  If that’s not practical, put them 20% of the way into the room from the front wall (7 feet in your room) and 25% into the room from the side walls (3.75 feet).  These are basic/general placement guidelines for a rectangular room from Todd Welti who’s a foremost expert on subwoofer placement.  The other option is to do “the crawl method” to find two places where the subs sound best from the listening chair and then set the volume, crossover, and phase as per these links…

https://www.audioholics.com/home-theater-connection/crawling-for-bass-subwoofer-placement

https://lifehacker.com/how-to-properly-set-your-subwoofers-volume-without-sh-1506136549

Just some thoughts and a couple ways to attack it.  Hope this helps, and best of luck.

My listening space - at approximately 18 x 15 x 8 - is markedly smaller (and less rectangular) than yours, but I faced the same problem. I, too, use Quicksilver tube amplification with a pair of Klipsch Belles - and struggled to find the best placement for a pair of badly-needed Rhythmik F12SE subs. (I realize this is all too big for the available space, but it’s what I have to work with. Plus, it makes for a nicely intimate "nearfield" listening experience). With the Belles placed along the "long" wall, I have each sub facing inward (across from one another) at the midpoint of each side wall (as suggested, I think, by @soix above). I experimented with them in the corners and, vice versa, on the long wall just inside the Belles . . . . neither of which sounded optimal. Since my "room" is a squarish formal living area that connects to the formal dining room and to the front foyer, my placement options were necessarily limited. Frankly, your Forte IV’s would be a much better fit in my space. Good luck!

I replaced a pair of excellent subwoofers (I will not mention the name/model) with a single RSL 12S (placed in between my Lascalas) and am thrilled with the coherence and dynamics of the combination. With your room, maybe a pair. Good luck. Always, MrD.

All your comments are great stuff, thanks!

@noromance - placing the main speakers on there long wall is not an option. Trust me, I have thought about it, including my wife's reaction! 

@souix - a lot to think about. I think the crawl method might be best. I have an opening to a stairs at one end and another adjacent space at the other end...It seems that is should be easier to find good placement for a listening chair rather than a listening area?

@tds3371 - I just can't get my head around the mid wall location. Probably need to better understand the physical properties of the sound waves and nulls?.

@mrdeci - Interesting that the one sub woofer is doing you good. Can you share why the new single sub is better than the two? Just too complicated with multiple subs for your space? Or the new one is just better fit to room? I was wondering if I should just start with one. I suspect that it would be am improvement, but would/could I be missing with two? Although most of what I am reading on forums is telling me two or more.

Do any of you have opinions regarding musicality of different brands of subs?

Do you think mating subs with horn speakers is more complicated than with non horn speakers? Read about that on forums and seem to be equal camps based on what I have read on this one.

Thanks!