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

"You can't fix what you can't measure" is what we say in QA.

Get a calibrated mic and Room EQ Wizard software (it's free!) and measure your current system.

My guess is you'll be appalled.  Most systems sound TERRIBLE in actual rooms.  (Check out the in-room frequency response graphs in Stereophile reviews for further examples)

Experiment with speaker placement.  Sometimes small movements make big differences.

My answer is "digital room correction" - my music is PC based and I use the free MathAudioRoomEQ plugin for my music server.

The Lyngdorf is a hardware based system that does similar.  There are others, including Dirac Live.

I'd at least audition some of these.  Measure the results with your mic and RoomEQWizzard software.  Decide if the trade-offs are worth it.

For me there's no question about it - I rent and have hard limitations on which room the stereo can be in and how I can treat said room.

You may find your answer through dedicated subs or, if you're lucky, better speaker placement.

It could be your room, you know. Read F. Alton Everest texts to learn about room modes. If you have a square or other poorly proportioned space no amount of gear at any price will solve this. As suggested by others, change your listening position. Mr. Everest will help with that and you may need to move more air with a sub. 

I haven’t read though the entire thread but, try high quality room treatments. I highly recommend the Stillpoints Aperatures.

Still, in this day and age, most people would rather trade a component for another at a loss/on top of a big extra expense instead of taking the time to try room treatments. Some have mentioned trying a Lyngdorf. My feeling is to get as far as you can go passively.This is the best path.

I am having the same problem with my b&w 800d. Suspended wood floor with 13/32" subfloor of particalboard. Crappy. Need to decouple speaker from floor. Spikes will couple the speaker to the floor. Isoacoustics have  newer product - footer that  will help decouple. Like the nordost sort fut  or some stillpoints options. Also townsend seismic platform.

Hello,
          Your request is not possible with the speaker you have, period. I have PMC speakers which are transmission line design and have great bass at lower levels but to get an increase at 28hz is unrealistic. I use a PMC twenty5.sub for foundation level bass below 60 hz. It blends in perfectly. Don't waist your good money on tweaks. 
Happy listening.