Greater displacement depends on the woofer's size and xmax and has(IMO) nothing to do if the loudspeaker is a basscab or PA speaker.
The drivers in my cab are the Oberton 10B200 and 8NCX.
Yes, Vd does depend on surface area & Xmax, correct. And you're also right about it having nothing to do with the box application.
But a single 2-way (w/coaxial 8) cab, while you (or whomever) may love how it sounds, isn't going to cut the muster in much other than a small to small-medium live situation. The 10B200 has low sensitivity (93dB) & being a 10", even with good Xmax (5.75mm), doesn't move so much air (182.5 mm). The 8NCX's sensitivity is 95dB, a tad on the low side. Add the 8NCX's 95.95 Vd (20.2 m2 x 4.75 Xmax) the cab has 278.45 mm Vd. The 2 drivers (taken from Lautsprecher-Technik) are 324 Euro (about $440 U.S.).
The similar bass cab I referenced with a 15" Eminence Kappa Lite 3015
LF (and a 6.5" 18Sound 6ND410) would have a minimum VD of 845mm. The 3015LF's sensitivity is 99.8dB. The 6ND410's sensitivity is 102dB. The 3015LF is priced at $240 U.S. & the 6ND410 is $146 U.S. (total $386 U.S., ~286 Euro).
This specific bass box will be larger & heavier than the P.A. box, but still very manageable at about 20" wide x 30" tall x 16" deep & ~47 lbs. It will also max out at 8kHz, but will cover the vast,
vast majority of bass players' needs.
And an alternative cab could employ either an Eminence 3015 (
non LF) or Faital Pro 15PR400 & Selenium D220Ti High Frequency driver (to 20kHz) & HM25-25 horn. With the Faital, the total driver/horn price would come to only ~$320 U.S. (~237 Euro). The Vd on this cab would be less, ~463 mm (vs. the P.A. cab's 278 mm). And sensitivity would be 100 dB minimum.
Again, if the P.A. rig is tonally what someone wants, great. And yes, it'll work. But all things considered, I don't really see any contest.