Bass response is so much more than the speakers themselves. Placement relative to listening position, and room relative to same are critical elements. Getting the bass "right" at the MLP, is in my opinion the most significant thing in this hobby. It is very much worth the effort to learn how to accomplish this.
To that end I'd be of them mind to add a sub with EQ to your existing setup. But really you should measure what's going on at your listening position so you know where to focus your attentions. A Umik1 & REW would get you there..
Others are right about the limitations of driver vs cabinet size etc.. but as mentioned above there are speakers like Golden Ear's Aon that use passive radiators to effectively increase cabinet size and bass response.
One thing a lot of audiophiles either forget, or don't know is that as the frequencies get lower, our ability to hear distortion decreases proportionately and also we perceive them to be quieter relative to the rest of the spectrum. (Search Munson curve)
If a speaker like the Aon is rated +/- 3db from 38hz & up it doesn't mean there isn't effective bass below that frequency, and you could probably not physically hear between 5% to 10% distortion down there anyways. The room itself can create as much as 30db swings through the audible range and usually the biggest swings are in the bass frequencies.
IMO getting the bass right creates the clean canvas for the rest of the spectrum to paint on.
