I power my 21's with an FP14k, nothing less would do. I have to turn my 14k down to keep it from clipping. But mine aren't Mach5's nor ported nor have I ever owned an inuke. (I have played with a few inukes in-person, but never hammered on them nor played them side-by-side...) YMMV.
On that note, most of the subs tested on data-bass took the full 20kW, but only for milliseconds and only above 40hz.
Even the lowly HT-18 V1 if I recall... which is like ~600w or whatever.
High excursion "car" drivers are very inefficient at mid-bass, so even with all that power all it does is heat them up, for the most part...
Now the bigger subs, such as the 21's and 24's, do have somewhat-decent efficiency, but still not like ~100db/w/m or whatnot...
Depends on how much of a basshead you are and what type of frequencies to want out of it (and how loud).
There are always limits.
Most people don't even know what they want, they think they want more 1-20hz, but most are happier with solving their >30hz firstly. Which can be done cheaply with a handful of 18" or 21" Pro woofers.
When it comes to <20hz, almost no amount of cone-area or excursion will be sufficient. That's just the physics of it.
A dedicated 600V 3000amp 3-phase breaker is almost-enough... hehe!