K_rose - maybe I'm missing something, but if you have reflections between two walls you should have amplification on even reflections 2, 4, 6 etc (in case of 5.25m it's 33Hz 66Hz 99Hz etc) and nulling (valleys) on odd reflections 1,3,5 etc. (frequencies in-between).
You should have the same amount of bass - only points of peaks and valleys will be different. It is possible that peak at 33Hz is useless (lowest bass E=44Hz, lowest piano A=27Hz but seldom used) and 49 Hz is suppressed giving you impression of weak bass (lack of extension). Damping room at low freqeuncies is not trivial and deep pattern foams are expensive and ugly.
You will get the strongest bass with speakers and listening spot at opposite walls.
The only speakers that don't do peaks and valleys are bipolar speakers (planar, electrostats) but they have modest bass to start with.
Try to toe in speakers at almost 45 deg angle - it will even up bass a little, will probably narrow the image but it will widen sweet spot. Room mode calculators are often very complex (3d plots) and might not be very helpful. Best bet to see what is happening would be to get good sound level meter and test CD.