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

There are a lot of tradeoffs in designing a woofer system and some of them may be in play in the differences you hear between the Merlin and the ATC.

For example, the Merlin is 89 dB efficient and the ATC is 85 dB efficient. The higher efficiency of the Merlin comes mainly from its woofer having a more powerful magnet. This more powerful magnet actually tends to reduce the level of the bass (and even lower midrange) region relative to the rest of the spectrum. If the Merlin’s woofer is then placed in a cabinet that is tuned fairly low, there will likely be a "sag" in the response curve above that, and this "sag" can extend up 3 octaves or more above the port tuning frequency.

If this is indeed what’s going on (or at least part of it), imo it’s the sort of thing that EQ can help with.

Duke

Actually, one way to embellish and raise output of the Scanspeak 8545 mid/woofer relative to the Esotar tweeter is to add a Scanspeak bucking magnet. This doubles the 8545's magnet structure and raises output by several db-- as suggested by Tony Gee of Humble HiFi, who did this with 8545s in some project speakers.  I followed him with this mod in my VSMs.  Another improvement is to power the BBAM with a higher quality +/- 12V LPS than the batteries or AC supply in the stock unit.