What is the most dramatic way of increasing a speaker's Bass and Low mid?

Moving your speakers closer to the corners will load the low frequencies and increase their volume.  Too much and it will muddy vocals and destroy the stereo image.  For a 36" tall speaker in a 12' X18' room with 8' ceilings, start with theses numbers to increase the bass: 14" from wall to speaker face, 22" from Woofer center to side wall. The next position, pull the speaker out to 22" and or pull it in to 27" from the side wall, or both. Each will reduce the bass a bit.  These numbers aren't 'woo', they are basic room acoustics - physics. Pushing the speakers to the walls and corners increases the room 'loading' of the woofer, the ratios minimize standing wave (eigentone) reinforcement. You can also download a spectrum analyzer from Google Play like 'Spectroid' and run pink noise through your system to get a rough idea of the frequency response for a given mike location.  But, by all means, listen.  The tools and equations are there to inform your decision, not make it.