<font color='#000000'>You are running into the Fletcher Munson curves! That is, the ear is less sensitive to low frequencies at low levels. A loudness control might help, or turning up the bass a bit at low levels.
I would add a subwoofer rather than get the bigger speakers. For less than the difference in price between the 40s and the 100s, you could get a very fine subwoofer from a number of companies, including Paradigm, PSB, Mirage, Hsu, and SVS which would outperform even the 100s in the deep bass.
As well, the best place for the bass is usually not the same place as the best place for imaging. You can place a subwoofer where the bass response is best (usually near a corner) and put the main speakers where the imaging is best, usually away from the walls. Corner placement of a subwoofer requires a crossover below 80 hz, but the 40s actually go quite deep in the bass and so give you lots of leeway in setting the crossover.</font>
