Both SongTower and ER18 MTM were designed by the same guy, Dennis Murphy. They sound remarkably similar across nearly the entire audio range, except that the ER18 MTMs dig deeper, with an F3 about 35 Hz and with useful response down as low as 31 Hz. The STs go down to just under 40 Hz. I am familiar with both. I own STs, and have heard the original prototypes of the ER18 MTM at Dennis's house. I think they have very potent bass.
I haven't heard the Mini Statements, and I'd have to read the 3 page write up to see what the bass response is. I trust you to do that
. I can't imagine dual 8" woofers producing puny bass.
I think one of the biggest differences will be the ER18 MTM is a standard front-firing monopole design. The cabinets are 13½" deep (plus a little extra depending on the size of the plinth you build). They can easily sit with the rear-mounted port less than 2" from the wall behind them without impacting the bass sound. Dennis and I deliberately did that to see if it mattered. We couldn't get the cabinet closer to the wall, and if anything, it may have sounded better that way. That seems to be an advantage of the mass-loaded transmission line cabinet design, at least those designed by Paul Kittinger.
The Mini Statements have a mid range with an open back, similar to the original Statements, the Salk SoundScapes, and the Philharmonics. You can vary the amount of stuffing to adjust the balance between that driver's front and back waves, and adjust the depth of the resulting sound stage. To take full advantage of that, you probably have to place them so the back of the cabinet is at least about 12" to 18" away from the wall behind them. Further may be better. The front of the cabinets will be another 13¾" further out into the room. They will also need some room to the sides, so in a corner or immediately beside a cabinet may be less than optimal. That's an educated guess on my part, but I do think it would be a waste to build them and be forced to place them close to the wall behind them.
Dennis Murphy might be a good person to ask about this. From his work designing the Salk SoundScapes and the Philharmonics, he has a lot of experience with the room placement of his open back designs.