The "physical pressure" exerted on a surface by a sound wave is miniscule, and near zero unless the SPL is extremely high. For example at 120dB (threshold of pain) sound is only exerting 20 Newtons per square meter! That is about the same as 0.003 psi. It's just not all that significant.
While I stayed current with his site for awhile way back when, his speaker designs, while obviously quite good, held no interest for me beyond linking them to folks that I thought might meet the needs of their apps, but otherwise have zero interest, so again, which L26?
In listening tests with fullrange drivers I preferred 80hz cut off for them.
That preserved the good time coherence of fullrange sound the best.
Concerning Ob Bass I liked perrymarshalls proposition
https://www.diyaudio.com/community/...boost-4-to-7db-w-passive-xover-no-dsp.404171/
But going for closed would be an option, too.
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Possibly? The two failure modes I am aware of for the first model Quads are: 1) crud somehow gets past the protective mylar and causes buzzing an/or ticking noises. The degree to which the noises are audible is heavily influenced by humidity. 2) Arcing. Given a powerful enough amplifier, sparks can cross the (narrow) gap between the stators and burn holes in the diaphragms. I guess if you have crud (situation 1) arcing might be more likely. I've never had arcing. My Quads were manufactured in 1964. I have had them for about 30 years. There are six panels in two speakers and every ten years or so one of them goes. You can get panels rebuilt by various people. It's a good idea to learn to install the panels yourself if you want too live with these speakers....
I used to cross the quads over low like that last time they were working. I used something higher for a full range driver. I'll experiment for sure. It's interesting to compare what perrymarshall did to what Nelson Pass did with the B5 - in the latter case you wind up with more going on in the bottom octave. I guess biamping makes it more practical to drive a speaker that way.