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

My friend and I preferred equipment that gave us a sound that was true to the source. So if you a looking for warmth, or what some might refer to as classic tube sounding, older Cary and Conrad Johnson amps/preamps would be worth seeking out. I once owned a Cary V12 amp and SLP-98 preamp and the sound was quite warm. It sounded good for a while but grew tiresome as the coloration hid detail. For me the Avatar most certainly had weight and body, and with Mullard EL-34s a nice touch of warmth. When I wanted transparency I would swap in Siemens EL-34s.

The Merlin's are easy speakers to drive. A properly designed and damped tube amp (say a factor of 10 to 20 which translates to about 1/2 to 1 ohm output impedance) will drive the speakers more effortlessly and exhibit lower distortion. Their smooth impedance curve does make them OTL friendly as well and Bobby used to show them with Joule Electra OTL amps.

After years of merry-go-rounding with electronics for a fuller bodied presentation without resorting to equalization I dug out my DIY JBL two way horns and bingo. Unfortunately the delicacy of imaging and sound staging suffered greatly.

My solution was quite simple and has adapted with all my subsequent speakers.

After re-reading your response your, "talking about the speakers themselves." Other than looking at the Stereophile Measurements response curve dipping in that region then quickly dropping off, I've got nothing. Good luck with that. 

Why not download REW (it’s free) and drop 100$ on a USB mic with a calibration file. Next, measure your system. Perhaps moving your speakers around may help, perhaps not. I suspect that your 6.5” drivers are just too small but you really need to measure. Look at nearfield response first which will show pretty much what the speaker is capable of, then measure the response at the sweet spot which will add the room response. After that you won’t be guessing anymore. 

Shall I suggest the E word: EQUALIZER!!!!

Or less radical moving speakers closer to walls and corners pretty much always helps boost the bass, but often at teh expense of 3-d sound stage and imaging so a dilemma there..

Equalizers....much easier, more adjustable  and to the point! If you need one you need one. 

I was an early adopter of the Loki EQ...don't use it much as I like to keep signals as clean as possible, but when I do use the thing it's great. The adjustment sweep is sort of "front loaded" meaning they do their thing with larger effect initially, but sensibly designed...mine stays in the preamp to amp line, on but out of the circuit...so it's ready! I've noticed it's amazing electronic transparency, and tested that by removing it from the rig and returning it with no audible difference...even with obviously longer cable runs to make it fit. Get one, try it, send it back if you can't make it work for you. Done.