FWIW, the reason I have never liked subs is not due to the accentuation of bass--a good sub will do that very well. It's just that I have never been able to integrate them without seriously annoying suckouts caused in the frequencies where they cross with the mains... I always end up with an obvious suckout somewhere, much worse that my in-room response without the subs. You might be surprised if you measure the in-room response with and without your subs. Anywho, back OT...
Long wall can work well, but short wall should be OK here, too. I have my ProAc D25's short-wall in a essentially 12x37' room (combined kitchen and living room), and I have very few problems with bass nodes/bass power. I do have some humps here and there, but in the real world we all do. I have the measurements somewhere... Can't find them now.
The thing I don't understand is how you would have such a broadband suckout such that all low and midbass up to 180hz would be suppressed. I have never had a room that did this, and honestly didn't know this could happen as most rooms I've measured have humps and valleys, most often related to "room nodes" that enhance or suppress the bass.
What I did tweak the setup on my speaks was setup a RatShack SPL meter and grabbed my Stereophile Test CD 1. There are steady tones from bass on up on that CD. I set the reference to the 1khz tone (I think that's the ref tone on that disc) and then measured the relative output at all the frequencies. I then had a sheet that when plotted looked alot like JA's in-room measurements in S'phile. I tried a few different configs, and settled on the one that had enough WAF and the least amount of issues (with nodes, imaging, soundstage, etc). Honestly it was pretty easy to dial in with respect to bass--the soundstaging proved more difficult.
One thing that was helpful to me was to position the speakers at slightly different distances from the walls (side to side, not front to back obviously!). This smoothed out the bass response a bit. But you essentially have that here...
HMMN. I also used a cool trial version of room-setup software somewhere on the web; I'm not sure if it models sloped ceiling, tho. I'll see if I can find the link.