You wrote ".. a real horn loaded speaker at 20Hz," and I’m telling you a 20cf. quarter wave tapped horn with a tune a ~22Hz will do honest and proper 20Hz - period. What isn’t proper is asking a smaller size, lower eff. direct radiator doing the same, even with a surplus of power. And horn sub iterations can be in multiples as well.
The realities of path length and flare angle means that for a practical sub-woofer, a vented box will always have the advantage at the lowest octaves. The horn is significantly more efficiency at higher bass, but at the deepest, it is not. If you are recreating the Saturn V, you need the energy at the lowest frequencies, 20Hz and below. You are better off vented. For pure music, you don't have a lot below 40Hz and dropping quick, horn is great for a sub. I realized the Saturn V was both hyperbole, but also representational. You want to recreate life's audible events.
A pair of corner placed (i.e.: with boundary gain) high eff. tapped horn subs and high eff. pro cinema main speakers with a combined 2.3kW actively per channel can shell out +125dB’s at the LP (~11ft. listening distance), full-range, so backwards math gives an easy 105dB’s with +20dB’s of headroom - within my actual required range.
That's fair. Essentially rock concert level, somewhat close to the stage.
I am glad you don't listen to it regular. As much as I love live music, I don't say yes to the frequent invites I get any more for amplified events and even for the last long while, I have generally enjoyed with ear plugs. I take enough of a "hit" professionally. Have to respect your ears.
The clean (and full-range) dynamic bandwidth not least comes in handy with Blu-ray/4K UHD playback of movies.
@donavabdear can comment on this better, but the target playback levels, we could call it the intended levels, are far below what your system is capable of.
10th row with a large symphony orchestra during tutties? I’d say 105-110dB’s.
10 seats out, 105 would be the max, and usually lower. If you were up on the stage, it could hit 120 with some pieces having extended 110db+ sections. This is starting to become a big issue, starting initially in Europe. Due to the amount of practicing, the musicians total exposure can be at ear damaging levels, especially in the brass section, even worse than percussion though percussion can have higher peaks. Lots of talk w.r.t. regulation, creating practice spaces with more distance between performers, positional changes to reduce total exposure, etc.