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

You will regret replacing the Avatar as I once did. I am not sure why it doesn't give you what you want, but in my own experience I sure missed everything it did well, and it did a lot well, when it was gone. Coincidentally I used the Avatar with Audiokiesis speakers. I also had a friend who was quite respected on these forums who powered his Merlin's with either Atma-Sphere M-60s or Music Reference RM-10 or RM-9SE amps. The latter two will give you low output impedance and fairly high damping, although not nearly as high as a solid state amp. All told though all 3 of those amps are quite transparent.

Im aware of the virtues of the Avatar so not in a hurry to part with it, however im speculating about my ability to tailor my sound mote precisely with separates. Especially a pre amp for tone. But gotta start looking at the correct power amp for the job first. 

What tonal balance was your friend after?
as evident from my post here im looking to trade transparency for weight warmth and body. So im not sure how the OTLs mentioned behave realy in that respect. How does the high damping translate to the Merlins?

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.