If you had time to get to know a pair of speakers well enough it isn’t difficult to realize changes in there high frequency character, they may start to lose crispness and detail and begin rolling off sooner then you remember. Said flaws can also make them sound like they dont play as loud as they did before.
If a problem lays within the tweeter layout you want them put back to factory specs otherwise you wont be happy with them. Even slight mismatch in a critical area or areas can kill what was once a strong romance.
Theres a good bet scanspeak can direct you towards proper replacements, with some luck. If not tweeters can be rebuilt if you feel the speakers are worth that time and expense.
If your certain tweeter performance has dropped off and the drivers themselves check out ok, another place to look is the inline capacitors.
This write up might be helpful..
Many, many speakers from the 1990 & 2000 decades had metal-dome tweeters damped with "ferrofluid." The idea was that the viscous fluid, magnetically held in the tweeter voice coil gap, would dampen resonances and ringing in the (then new technology) metal-dome tweeters. Now, in 2013, many of those tweeters sound dull, or have quit working altogether. This happens because the liquid in the ferrofluid has evaporated, leaving a muck or solid in its place. The muck reduces amplitude of the tweeter (making the speaker sound "dull" or "muffled"). The solid prevents the tweeter from working at all.
Most people throw away their speakers at this point and buy new ones. But with a bit of elbow grease, the tweeters can be restored to "like new" performance. This thread shows how.
Most speakers have removable grilles, but the ones my friend gave me were fitted with "socks" that were secured at the top with a tied-string closure. To drop the socks, untie the string. Then, at the end of the string tips, tie a knot. Why? Because if you don’t the strings will retreat into the top loops in the socks and you’ll never get them out again (I speak from experience here...).
Once the socks are gently pulled down, unscrew the tweeter faceplates from the cabinets. You may need to wiggle the tweeter edges with your fingernails to get them to come loose. The manufacturer of the speakers I was working on (Paradigm) inset the tweeter faceplates to be flush with the cabinet face.
After removing the tweeter from the cabinet, stop and mark which wire goes to the + terminal of the tweeter. Paradigm made it easy for me: Blue wires were + or positive and green ones were - or negative. They also (thoughtfully) supplied a + and - molded into the back of the tweeter’s faceplate, so I didn’t have to mark that either. Had Paradigm failed to do these things, I’d have needed to use a marks-a-lot to mark one of the wires (wouldn’t matter which) and then use an awl to scratch the matching terminal on the tweeter. Why not just mark the tweeter with the marks-a-lot? Because solvent will be needed to clean the tweeter, and when applied, it will wash away any marks-a-lot on the tweeter.