Hmm, don't have access to my 421's measured specs, just recall them being somewhat different than published, requiring a larger cab, so in a small sealed cab, it will probably need a ~aperiodic [damped] vent to basically make it a leaky sealed cab as I'm not sure enough damping will fit, though know at some point it 'sucks the life out' of wide BW drivers and probably why heavy stuffing of Altec or similar woofer designs was only done with < 500 Hz XOs.
No experience with the JBL or any of the others you listed, though the 421's 'chrome dome' will likely need damping/replacement. Considering the 2404's 3 kHz lower limit, mating it to a 15" seems a very poor match.
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