I owned the Advents for 27 years. I think it's mostly how the speaker was designed rather than the age of the tweeter. The soft dome that Kloss used didn't have the edge of some tweeters of that vintage, and is certainly more rolled off than most tweeters in modern speakers. I never found the Advents dull, but they never had the high frequency extension my current VR-1s have, even when they were new.
I've read of folks having success replacing the Advent tweets. You might do a little searching on the Web, or contact someone like Madisound.
I bought my Advents when I was also a poor student, and they travelled all over the country with me. Even though they're not to everyone's taste, they're definitely classics.