No room has flat frequency response. There are always peaks and dips, but one is easier to deal with than the other.
Take for example bass frequencies of 100Hz and below. This area of the music is of critical importance to get as flat and accurate as possible. Yet, the room has other thoughts about flatness. Standing waves, absorption, reflections, and shape all play into forming the peaks and dips we're all so familiar with.
If you were to add a multiband equalizer to the speaker generating those lower frequencies you might fix half of the problem.
Peaks—extra energy—are easy to tame down. Find the frequency region, set the equalizer up correctly, and bingo! All that extra energy is squashed down to flat.
Dips—areas of sound reduced in energy—are the opposite: almost impossible to pump more energy in to the original source because the cancellation causing those dips is powerful indeed. Nearly always, no matter how much more energy you throw into that frequency range, there will remain a dip.
But, there is hope and that hope depends on placement.
As I mentioned, no matter how much energy you add to the original source of sound, you won't get anywhere. Move that source of sound (or add another sound source like a subwoofer in a different area of the room), and voila! The cancellation occuring at your seating position vanishes.
Brute force works sometimes, but other times, a little knowledge and finesse go a long way towards success.
