Speaker upgrade

Bass, Bass, Bass, Bass and Bass and more Bass.
Get three subs (three is better than two in the sub department) and if it's not enough rip the tweeter and put a woofer in it's place.

Ok, we want bass, but the bass that exist in the music, not coloration. We dont need speakers that play below the 30 Hz, instruments dont go there, unless you hear organ music. Even if your speakers go to 25 Hz 0 dB, thoes your room has space to give this frequencies.
I'm not an adept of a sub, if you want better bass change your speakers, dont buy a sub. Inplementing a sub in a audio room is difficult, and most of the time the results are only satisfatory.
The question is - What do you want? Better bass; or as someone put it "room filling impact without boom."

Warrenh, could you please explain the concept - Power plus room nulls avoided - when two subs are used.

Warrenh we have something in common, I'm a sushi fan too.

Sound waves are mechanical waves because they need a medium to propagate their interaction. The main properties of sound fields can be deduced from the basic association that the perfect gas is an ellastically shapeable and mass-adherent medium. The easiest way to study the sound field is by studying it's pressure field.
Every wave as nodes, sound waves too. The distance between nodes give us the pitch.
When sound is propagated in the resulting sound field appears a series of nodes. When we have two point sources one can make use of the superposition principle: If two or more traveling waves are moving through a medium, the resultant wave function at any point is the algebraic sum of the wave functions of the individual waves (due to the linearity of the wave equation). In the resulting sound field the nodes appear where the waves interfere in a destrutive way. The position of the nodes is dependant of frequency and phase. The fact that we have two point sources dont make the room nodes disappear, at minumun they can move (in the time domain or in the spectral domain).
In the case that the point sources are two subwooofer we can make a simplification in calculating the resulting sound field if we assume that the ratio between hearing position/wavelength <<1 ; the two source strengths can aproximately be added; at low frequencies the sources act as if they were put "in the same place" - thats why bass frequencies are not used to build the stereo image(lack of directionalaty).

Saying that one must go to the AudioAsylum can be useful (but not for homework), but using the information in a way that we dont understand is another thing. When giving an opinion we must try to be the most rigorous (Maybe my problem for being a scientist).