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.

There are tons of interesting ideas on this thread.

But I'm going to reiterate: you can't fix what you can't measure.

Get a USB measurement mic from Parts Express.  Use it, and post a FR plot.

Let's see exactly what we're dealing with.  Then you'll also see what your improvements do.  It's possible that relatively minor adjustments in speaker placement could yield the improvements you're looking for.

Ears are great and all, but the placebo effect is all too real - I worked in Pharma QA, and it is frankly terrifying.  It is only now, more than 20 years later, that studies suggest Prozac MIGHT work better than a placebo, for example.  The analogy to various audio tweaks is left as an exercise..

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.

I'm still in the flip-phone era, but yes, this is a GREAT way to add measurements to ears.  (Google says there's one for the iPhone, too)

Srsly, this is cheap/free (the PC-based REW is free, dunno about the phone apps) and should be what you start with.