I agree with this approach, especially for DIY. Modular construction is easier in several ways, and it allows swapping just one section at a time.
It's worth taking a look at
Augerpro's waveguides. The SB26ADC works
really well, and the price is right. What we need is a list of CNC/3D-print services that are reasonably affordable & friendly to small-volume projects. There's Speaker Hardware and?
Pretty bonkers, yes. While I like sealed woofers, full-range performance bumps into both Vd & Hofmann. All dBs from VituixCAD at 1M in half-space.
The PTT6 has Vd of 130cc, and a 15L BR/PR version can do 105+dB in the passband with f3 around 40Hz. Put a woofer with 500cc Vd - say an RSS265HF-8 - in a sealed box, and you get 105+dB in the passband with f3 around 40Hz from 35+L. Well, crap. If room gain cooperates, the slower roll-off could make 20-25Hz useful, but it's hardly an improvement at 30+Hz.
So, you're right about needing to go
big if you want a sealed box to outperform significantly the R1 in the bass.
Let's step up to the 15" RSS390HF with >1100cc Vd. (And I'll strongly encourage RSS over UM, especially when using higher XO.) If you want to keep the box reasonably small, say 55L, you get Qtc around 0.9 & need 500W to push near Xmax. An 85L enclosure will drop Qtc to 0.8 & allow Xmax with 300W. Go even bigger, and power requirement drops while output stays the same. In all cases, you see 105dB to 25Hz, so f3 reaches 20-22Hz. That's expected, as 1 octave = 4x Vd.
Considering that Rick & Amir don't want to deal with a massive box, plus the benefits of distributed subs, I'd say that ~55L with an RSS315HF-4 or ~700cc equivalent is about the practical limit. That will deliver 105dB above 30Hz with a 32Vrms amp, such as the NC252MP. External dimensions could be roughly 12" x 18" x 24".