I can think of one design that does exceptionally well and is as flat as a board to 20hz with around 150 watts wired at 12ohms. It has everything to do
with the cabinet design.
GRs design not only works well it has for close to 20 years now. FLAT baffles in the sub-region have a definite disadvantage because of the width and directivity
of the baffle. GR's happens to be a servo TOO, which mitigates the overshoot when returning to the neutral position. It's one of the reasons for reversing the drivers
in every other position on the baffle and a modified baffle step that is increased considerably MORE vs an IB or ported design, whether with actual ports or a passive
radiators.
The one really good thing when adding a waveguide in BOTH directions, is the folding/canceling is VERY limited, and because of that, a 370-watt plate @ 4ohms
and around 150 watts at 12 or 185 @ 8 ohms will fill a considerably sized room. It is also a LOT easier on the ears because of the way the room is loaded,
especially when the room is sealed.
The GR drivers are pretty efficient. 90+ at 8 ohms. You can stay in a 3-400 sf (8 ft lids) room for protracted periods even with extremely bass-heavy tracks.
The Mechanical Q has to be above 50% for either servo or non-servo drivers to work at lower frequencies BUT a wider baffle works where a narrow drops off
no matter the baffle step.
Even though there is some cancelation, it's limited to a given circumference, BUT mainly on the sides for about 24-36" There is a dead zone so to speak.
I use 4-8 (total) 12" drivers with a pair of Bass Columns. The lower I go with the servos XO point (to a point of no return) the better. They are
set at 50-60hz < 24 db. A fairly steep slope where the Bass columns are 48db on the bottom at 50-60hz and 12-24db on the top at 280hz. The mid-drivers
are at 24db also. I prefer a SHARP tight response and a light quick recovering cone 8" or smaller and in multiples like an LS design for the bass columns.
Personally, my favorite is a graphite cone for bass drivers.
I have a combo that works extremely well. The columns throw a center phantom when they are set up correctly and the arrival time is mitigated by
mechanical location/placement, NOT by electronic delays from DSP. I do sometimes implement a notch for ceiling height if the ceiling is not treated.
If I remember there is a suck-out below 50 and 80 but a peak at 50 and 80. It's for 8 and 10-12 ft lids. So the -5-8db and the + 4-8 are taken care of
by PEQ if the ceiling causes an issue you can notice. I notice it when I play classical guitar tracks with a lot of congas/bongos in the mix.
The columns in the middle are SAT (stack and test) and they are made to go up to 9 drivers or 3 cabinets tall for 12+ ft lids.
I also prefer phase plugs to help with back wave return/cone (distortion) The drivers are Dayton RS225-4. I think they are static wired at 12 ohms
They are direct-coupled to Behringer NU12,000/DCX2496 using # 10 PTFE OFC. VERY well behaved even at 100+db. Cabinets are triple-coated with
@Soundcoat inside. DEAD quiet cabinets. Like knocking on a water gutter in front of your home. NO, ZERO, NONE on the knock REPORT!
BTW ALL the drivers are very close to 90% efficient @ 8 ohms or better. You don't have to adjust volume because of the difference in efficiency between
the subs, bass, mains (mids), and tweeter columns. The mids and tweeters are mounted to the same baffle. I do have a few rumble filters, that I use
for LPs though.
I decouple all the columns, usually air ride BUT I did design a good adjustable accumulator/cantilever system for HEAVY cabinets 400-1200lbs.
1.75" @Corian baffle, 1.25" HDF cabinets 84" tall. 14X18X84. 625lbs. Small planar/ribbon cabinets.
The outside cabinets below use GRs 12" 16-ohm drivers in pairs wired at 8 ohms. I use 2-4 (doubles) depending on the room volume. The cabinets
are by CaptainNemo and A/B plate amps are by Rhythmic. The Bass columns and the Queen Ann legs are my own, made by an analog duplication
machine, I managed to wear out over the last 50 years. I did get a new one though.

No, CnC in my shops, BUT I've been thinking about it.
My personal opinion about full on open baffle is "THEY SUCK" but after 25 years with Infinity IRS Betas off and on, they are a lot of fun, just not
very accurate.
Regards