This room for 2 channel or HT?
There are many "room mode calculators" on the net(www.guidetohometheatermag.com, etc) if you look.
For fast reference, see the July issue of Stereophile Guide to HT (I think?)'s "home Theater ARchitect section" (otherwise, maybe it's June...not sure). Anyway, this should help your thinking greatly!
Basically you want smoothest bass bellow 300hz you can get, sepecailly in the lower registers. No "stacked up peakes" is a major help.
Basically, you want to move that back wall only if it helps smooth the bass. So you need to find which exact dimmension is best from what you have to work with.
If I were you, I'd consult(for a few hundred bucks it's well worth it) for room dimmensions, seating and speaker placment, as well as acoustical treatments.
Your other problem will be "too much bass reverb, and lack of absorbtion "down low"! Too much bass energy needs to really be "absorbed" with large bass traps in that room. If you can't do it in the ceiling, then probabaly a large absober in the back of the room (properly designed to allow diffusion for midrang/trebble) is what you need.
There are many acoustics books, articles, and helps if you research. OTherwise, pay the few EXTREMELY VALUABLE DOLLARS on the single biggest investment/improvement you can get!
A couple of the big players are PMI, Rivesaudio, maybe RPG or ASC, etc. These companies could easily(I do this work personally) engineer your room quickly and maximize sonics. (figure cheeply at $750 or so for basic consulting, yet effective as heck).
The difference between improper dimmensions is "stacked room modes" and "gaps", making seating and speaker locations that much more unmanageable for perfection. The difference in "not enough bass absorbtion" in that tiny(yes, acoustically dinky) basement room is you have a thick, overly warm, extended reverbing bass response, that's slow, lingering, and dominating the upper frequencies. Bass is slower, less impactful, clean, accurate, etc in this case. Compared to a large acoustic space (like commercial theater)or well engineered room is potentailly world class bass, due to massive surface absorbtion, multiple seats/boddies, floor construction(also surface), and absorbers(which theaters offer). In short, most peoles small rooms sound like "pressure boxes of boomy bass with no speed, impact, articulation, definition, or balance.
Pay the bucks and get teh best...that's my recommendation. Otherwise you'll end up with mediocre with lack of knowledge/experience