I had a similar problem and my room is 14x22x10 ft. I have a 40-50Hz suckout that can be 15db at its worst ( ctr of the room). I got it to 2-5db (depending on smoothing of freq curve). Many reviewers have a similar issue, just take a look at the measured room response on stereophile. John Atkinson constantly mention this when reporting his measurements in his room. I suspected he left the problem there to simulate rooms of most readers.
More info on room size, sitting location and speaker location would help. I'd guess that you are sitting fairly close to the center of the room which has the null. Hopefull, it is a narrowband suckout. I would also guess that you have peaks above and below the suckout.
One way is to sit closer (but not too close) to the wall behind the listening chair. This brings in some boundary bass reinforcement to counteract the suckout. Trade off is comb filtering issues smearing the midrange and treble but this can be reduced with traps/absorbption or diffusion. I'd position mic at different locations of the room and learn the frequency response. Do not forget vary the height of the mic slightly as bass response also change with height.
Another way is to move the chair futher up pass the center of the room and get out of the null. This probably would yield a nearfield listening location with great clarity. Also try using room optimizer software to generate a few sets of theoretical speakers/sitting locations to save time.
Room treatment and attention to acoustics is a must. People talk about how great a seal box design can generate clean/well damped bass but if room has many issues, the advantage would be lost. Conversely, ported design can work well in treated rooms.
I also have a Bagend trap. This only works 20-65Hz and tames peak only. There are just two bands with adjustable Qs. When taming the peaks above and below 40Hz, sometimes it can recover energy at the suckout point. This did not happen in my room. I have a 5db peak at 25Hz which the bagend can reduce it. I usually don't turn it on. On most music, this not too bad. The peak become noticeable on sustained bass notes (organ) and it loads up the room.
I also tried the subwoofer route. If I am at the null point, no sub can generate enough power to overcome the suckout. Outside of that, the sub can indeed compensate for the suckout and get a ruler flat response. There is a need to use some type of notch filtering (high pass filter with adjustable low cutoff). Otherwise it would worsen the peaks around the suckout. For me, it introduce group delay issues in the time domain. I think that if you go the subwoofer route, active crossover would work better. Some parametric equalizer would let you implement crossover and equalize at the same time. Problem is that they do not address reverb issues.
When you are done with freq response, optimizing RT60 would also yield big improvement in sound.
Good luck. I am just learning. Since we have similar problems, I 'd like to share what I went thru. I highly recommend room optimizer software, fuzzmeasure and room treatments. All are cheap comparing to two quality subwoofers or equipment overhaul.
