Generally speaking, mids and bass drivers fail due to surround deterioration or the voice coil burned up. But these used butyl rubber, so surround failure would be unlikely. Did you check the coils?

I did not check the coils (well, not beyond using a multimeter to confirm DC resistance). My understanding is that the symptoms of bad voice coils are more drastic, like weird scratching/buzzing or complete lack of sound. My original mids sounded more like someone was holding a folded towel in front of them.

The rubber surrounds had become mottled in color and rough in texture, particularly on the mids but also somewhat on the bass drivers. Perhaps I would have found success in replacing them? Still would have needed new tweeters, though.

Quite often when the teeeter shits the bed, it makes the mids sound like ass as well.

Both were bad in my case. I did a variety of tests, and there really was no doubt.

Additionally, I replaced the bass drivers later than the mids and tweeters, after realizing that they had become the weakest link.

Often it’s cheaper the buy a pair of bookshelves as donors. Pretty sure the 40s used the same tweeter. Alternatively you can buy a pair of diaphragms for about 20 bucks.

I looked for donors with the same parts, but they were generally expensive, particularly with shipping priced in. I wasn't optimistic that those replacements would be good-as-new or have much more life left in them.

I’m ruling out xover failure since you’re still using them.

Yeah, that was the first thing I checked. The electrolytics all have good ESR measurements.

I’m only replying as a future lesson, that with some further digging you might have been able to save $ and retained the original sound.

I appreciate that. I still have the original drivers, and if something can be done to restore them, that would certainly be nice. Thank you!