There are similar builds by Chop and erwinfrombelgium, but this one will use different mid-bass drivers. I plan on using the same 450hz XO and the MTM arrangement.
The AE drivers (8ohm with black phase plug, non-apollo) have already been purchased, and I've heard the lead time from AE can be a little while, but I’m in no rush. I need to sell my current mains before I have the cash to purchase the BMS drivers anyway.
Since I already have a miniDSP 10x10 and enough amps, this will be all active, with the exception of the 6,300hz crossover for the HF driver in the BMS coax.
The 18s will be ported with a slot port, with a tune somewhere from 35-40hz. They obviously won’t be completely full range, but most of the time they’ll be crossed at 80hz anyway. However, with a ~35hz tune, they should do fairly well for most music without subs. WinISD simulations seem to show that anything over 5 cu.ft per 18 isn't beneficial, so I’m leaning towards that number for box volume for each 18.
Similar to Chop’s build, it will be modular, but with this one, even the SEOS will have its own box completely separate from the 18s. Since Baltic birch isn’t cheap, and this build will be modular without weight concerns, I’ll be using MDF with plenty of bracing and doubled up baffles.
Final cabinet tune ended up being 25hz. Crossover point is 450hz.
Final outside dimensions of the boxes:
Height: 29.5" (tops and bottoms are doubled up on MDF (1.5" thick))
Depth: 29.25" (front baffle is 1.5" thick)
Ports: (2) 11.375" wide, by 1.5" tall, by 8.5" deep ports per box (depth includes 1.5" baffle).