How far can room treatments solve boomy bass?

I tend to side with Zman here. The usual sorts of room treatments likely won't amount to much more for you than a band-aid type of fix at best. If it were me and not knowing your budget limitations, I might consider, until you can get a better room anyway, the short-term fix of breaking down and getting yourself some sort of equalizer...yes, I said EQ! Possible insertion losses aside (which I think are often over exaggerated, myself), they offer enormous flexibility in just this kind of situation - expect your bass problems to be solved and then some. Perhaps a nice Rane unit on the used market, or possibly a digital EQ if you only have a digital source - a Behringer DEQ-2496 can be used in a digital passthru mode and costs peanuts...very little signal degredation this way if you are using it between a transport and a DAC for example. There may be other possible candidates out there, you just may have to do some looking, or ask others here. Hope this helps.

I don't understand the concept that acoustic treatment for a "too small" room is not the answer. It's exactly (part of) the answer. The modal distribution in a small room is lumpy well up into the upper bass region. The larger the room, the lower the frequency where that lumpy response starts to even out.

If your speakers are so large that the drivers don't "integrate" within the listening space, then yeah, that's a problem, too, but a small room is the worst offender when it comes to acoustic response in the low frequencies. Fix those and things will definitely get better.

Most likely you have one or several modes.
You can look in to a way of finding out.
Buy a XTZ Room analyzer pro II. It is complete and it'll show you exactly what is the problem. You can also use a Rives Audio PARC. The PARC is easy to set once you have the Room analyzer pro II. You just use the settings that RA pro II will give you, frequency, Q-value and level and feed it to the PARC. If you need to rid such modes most stuff will not suffice. You can also use Svana (diffusor.com) V-6 and V-4. These will do but they are large and very very heavy. I have used all most all Gik Acoustic panels, nothing will rid those low frequent modes. Not even the Monster bass absorbers (4 units) is anywhere near doing what the PARC does.
I use 14 Gik panels now (242,244 and TriTraps) and a PARC (measured with XTZ RA pro II). Best results you obtain by using both acoustical treatments and a PARC. I can assure you, this will take you where you need to go. You should now, bass modes doesn't only affect bass, it affects the whole range. You obtain better clarity, presence and PRAT. No lumpy swollen bass.

Walls create bass peaks. They amplify frequency (and multiples) that is f=1135/(2*L) where 1135 is speed of sound in ft/s and L is distance between front and rear wall (2 is for traveling back and forth in order to add). Larger room should help but you can also use sound absorbing panels. Normally they work the best when they are spaced from rear wall by either 1/2 or 1/4 (don't remember) of the wave (eqv. to frequency amplified by room). In my room it is not even possible and I'm forced to put them on the wall. I bought John Manville 817 rigid 2'x4'x2" (faced) fiberglass panels and glued them to fiber boards. I will stretch fabric on them and hang on the wall. Look at sound absorption coefficients: http://www.jm.com/insulation/performance_materials/products/ci9_800series_spin-glas.pdf

At 125Hz it is 0.38 - still OK but less dense (and more common) 814 is only 0.24. It is basically density (weight) of the panel that is amount of fiberglass they used. 4" panels would be ideal but I don't have space for them and there is, of course, WAF. Forget about curtains, blankets etc. If you can hear bass thru it then it won't stop bass reflections.

You can get much closer to unity absorption (which isn't a limit) with 4" - 6" of fiberglass. You don't want to overdamp the other frequencies, though.

The reason spacing off the wall helps is because pressure doubles on a rigid wall while velocity goes to zero. As you move away from the wall, there is a tradeoff between pressure and velocity. No damping occurs with no velocity -- therefore, spacing off the wall (or more thickness) is needed. At lower frequencies, this leads to ~ 6" panels to get effective absorption. Bass traps in the corner become a viable (if unsightly) alternative.