So the main thing that has changed here is that power is extremely cheap. For sub duty we're down to 10 cents per watt or less on amps that are reasonably reliable new with warranty AND this is going to be stuff that's extremely efficient which will run cool and not require nearly as much electrical upgrades to properly feed.
With so much cheap power the subwoofer technology has moved more and more into things with enormous power handling most of which are designed to perform in a (usually large) ported box.
It is extremely likely you could take your 4 sub budget, buy just 1 larger cone higher power handling driver, put it in the same airspace, and outperform the 4 1990s tier woofers. The idea of packing as much cone area as possible into a car is rather long dead outside of some niche applications and results speak for themselves as to the superiority of fewer woofers in adequate airspace.
I would not expect a whole lot of life out of a 20 year old amp, the 20 year mark is roughly when electrolytic capacitors start to break down so your old RF may be in need of a restoration in the near future if you wish to use it going forward. I would not use a class A/B amp for sub duty under any circumstances these days and outside of nostalgia value they have nothing to offer over modern class D designs.
Anyway, what exactly are you trying to accomplish and how much space are you willing to sacrifice for this project in what sort of vehicle?
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