Thanks for responses.
I did install the new tweeters carefully, and they definitely are in correct polarity. I noted that the internal hook-up cable was labelled Bandridge Superflex 2.
The mid-woofers sound great, so it isn't the electronics.
I'm using Synergistic Research Signature 2/3, and I've hooked both the thick and thin runs of cable up to both the tweeter and the mid-woofer binding posts, individually, to confirm that everything's running as it should.
Basically, my questions boil down to this. If a tweeter is blown, or has failed from age, will it pass any sound at all, or can it pass a little "throttled" sound? Likewise, if a component in a crossover is going bad or had gone bad, depending on what that component does, can the mid-lows be left totally unaffected, while the highs are impacted in the way that I've described in my original post?
I have to confess to being baffled and frustrated from the results I've got so far. When I installed the new tweeters, I either expected no improvement (=crossover the problem) or 100% improvement (=solved!). Instead, I got about a 25% improvement. I'm still struggling to understand how this could be, unless Proac tweeters, when run on their own, sound very different from how you'd expect a tweeter to sound when not underpinned by the other drivers.