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

Does the speaker have loose specifications?

When you indicate that you want to boost the level of the bass drivers, that suggests you don't consider there to be enough punch, dynamic impact. What makes you think you will get worthwhile macrodynamics from 6.5" drivers? 

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 

dumbeat, check out Shunyata cables for starters. They put their data up for the world to see. 

If you want to read about evidence for God, start with Michael Behe's classic Darwin's Black Box. 

dgarretson, as usual your eloquent explanation is thorough and insightful. Nice to hear from you again! 

Prof, we may be talking past each other on your first rebuttal; I'm not saying cables have little influence literally on the sound in a studio. I am saying that the fantastic resources of the studio to manipulate the sound do not transfer, i.e. presence of mixer, to the home. The environments and tools to contour sound are radically different. In the home environment cables take on a much more important role in shaping the sound. 

As to your second argument, my experience with dozens of brands and many looms of cables is fundamentally different, which leads to my statement about sound quality of a system and cables. Consistently use of lower end, less expensive, etc. cables (and, yes, pro oriented such as Mogami) have been mediocre by a great deal perceptually. I just recently once again had that experience as I was testing out three different sets of Y cables, and the worst was the pro/musician brand, while the best (clearly audible) was the specialty one from Audio Sensibility.  It seems we will simply disagree on this topic. I would suggest that your perspective is a disservice to audiophiles if you wish to make it a declaration. I can add the phrase, "... in my experience," and then we are not pretending we have absolute knowledge. We  are at this point at an impasse, however I suspect we will continue to be cordial. :) 

dumbeat, your prodigious skills and knowledge regarding studio does not all translate directly to home audio - or else you wouldn't be asking for a relatively simple fix for an audiophile in your original post. Many of those with decades of experience in home audio would know how, and successfully obtain the result they wanted.

Note this; home audio is not as much a science as in the studio, but more so an art, due to the difference in equipment available. I get what you declared without substantiation in regard to cables being the last 1%, but take way all the toys of the studio and now what have you to contour sound? The very recommendations in regard to placement, room prep and cables that the experienced audiophiles are recommending.

Bill Dudleston of Legacy Audio does work inside and outside the studio. As an experiment he humored me in a review of his Legacy Whisper speaker; he upgraded the internal wiring and caps at my request in an effort to see what improvement might result. He had doubts, but afterward measured a 2 dB change in the speaker's bass. You know how significant that is. So, one recommendation I could give you is to consider upgrading the speaker's internal wiring and caps. If you don't like the suggestion of changing external wiring, perhaps you will accept the experience of a person who does work in the studio and found that internal wiring and caps made a difference. 

As far as arguing with the people here with extensive experience who are trying to help you, go right ahead if you feel you must, but you are rejecting the very means you say you seek in fixing the problem. Please sit down in front of your rig with some different power, IC and speaker cables, and do some critical listening. I have had some well known industry members visit my room and hear comparisons between cables themselves. Like them, you will learn something. Now, if you are unwilling to do that, then cordially I am finished with our discussion. 
:)