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

dumbeat OP
Cables are the last 1%, as we say in the studio world.

Steve Hoffman might have different opinion on that.
Brian Lucey also.

Translation: cables are the last 1% in the pro audio world. 🤠

Equivalent statement: sound quality is the last 1% in the pro audio world. 😛

It seems you're expecting a  lot from 6.5" drivers. A Vandersteen Sub-3 with it's complete bass equalization controls might give you what you need.

The ATC 25ASL PRO has a 7” woofer  And The low mids to liws are very very full. Easy for a 7” woofer, should be easy for a6.5”.  I have no issues with the very low area. It just aounds empty and hollow in the low mids. Its not the size, its the voicing. 

Doing a bit of armchair research is helpful; the speaker specs at 33 Hz
 +/- 2dB, tight specs. So, 28 Hz +/- 3 dB is not inconceivable. 

Then, there shouldn't be any "hole". So, I see the frustration of dumbeat, as it seems there should be enough low end presence. However, this is your classic tiny tower, and you simply are not going to get prodigious bass with smallish drivers in such  cabinet, regardless of the fine quality of a Merlin speaker. If you try to push such a speaker in an attempt to jack up the presence you are now flirting with distortion. Not an easy balancing act. 

It is telling that recommendation was to put a "BAM" unit with the speaker. I found this in an old Stereophile review, "BAM Bass Augmentation Module: Supplies 5.2dB of boost at 35Hz and infrasonic rolloff below 27Hz." 

It seems to me the BAM was a concession of the weakness of this speaker in regards to the bass presence region. Nice design in many respects, but problematic in the region dumbeat has pinpointed. To avoid such things one typically has to move away from skinny, smallish towers toward larger cabinets and bass drivers.

Note also that the ATC has a bass adjustment from -2 to +3 dB; again, such things are necessary when speakers struggle to produce "natural" presence in the mid-bass.  

As usual, size matters in both cabinets and drivers.  

Finally, a word about cables. Studio use of cables has little relevance to domestic systems and cables. The applications are vastly different, and the beneficial nature of working with cables in the home becomes apparent quickly (If one bothers to try).  An exceptional way to mediocre sound is to ignore aftermarket cables, and or stay with "affordable" cables to avoid comparisons and advancement. Having worked with dozens of brands of cables over decades not only are cables responsible for far, far more than 1% of a home system's sound, but the selection of cables can literally make or break the components and speaker's performance.
  YMMV