Seas Excel Millennium Tweeter

@b_limo, I think so.

The break in issues I've experienced have been with mid-woofers, woofers and caps.  Not tweeters.

@erik_squires, The crossovers look good, I thought about modifications but I like the speakers to much as is!! My "gut" feel was there would be minimal break-in on a tweeter, but never having to replace one, truly value other opinions against my ignorance.

@b_limo, good question on what made it go bad, no clear indication.  I noticed a couple pops from my Music Reference RM10 amplifier, one for each speaker, at which point I rolled a few tubes.  It was shortly there after it was very clear that the tweeter was not working. Also in the circuit are a Tom Evans phono and a Blue Circle Attenuator.

Erik is right.  It could be a problem that occured within the crossover.  Take out the woofers and examine the crossover circuit for any burned out resistors/caps.  Although, it could be that a component is burned out but may not have any visible signs of damage.

Also, before you go buying a whole new tweeter, see if you can take apart the tweeter itself.  You can buy a new voicecoil/dome part for the Millenium for only $52:

https://www.madisoundspeakerstore.com/seas-voice-coils/seas-h9929-voice-coil-for-t25cf-002/

When you have the tweeter out, it is just removing those 4 screws in the middle.  If you can pull apart the assembly, then you can just replace the voicecoil/dome part.  If you cannot pull it apart, it could be that the voicecoil melted onto the magnet.  In this case, you will have to buy a whole new tweeter ($322).

Before you go off doing this, I would verify that you do not have a bad crossover in that speaker. 

1. Take out the supected bad tweeter and put masking top on it to mark it as the "bad one".
2. Take the good tweeter out of the second speaker and install it into the first "bad" speaker.

If it plays as expected, then you do have a bad tweeter.  If you get a problem with the sound, then it's likely a bad crossover.

@auxinput, Thank you for the troubleshooting tips.  I did not think about simply replacing the voice coil!  I am an electronics technician with a fairly complete home bench setup, I have verified a "bad" tweeter, what I did not verify was if it is simply the VC, HUGE Thanks for the advice!!

Really nice speaker, but a  bit big/heavy for my taste,,although am considering down the road to acquirea  pair,,,I have the Seas Thor MTM , has the Millennium tweet, I asked Richard Grey should i replace the 17 yr old tweets, *do they work? * *Yes*  *Then  no need to replace*.
I am considering upgrading my Millenniums to the new Flagship Cresendo tweet,,,and the xovers to all Mundorf supreme caps. 
back to you.
You could ask madisound if you can use the Cresendo tweet/replace the Millennium. specs look near identical, except the Cresendo is a  high sens. Like 93 vs Millennium's 87 =- might be a  issue. 
If not, just order the single Millennium, plug and play, well might needa  tech to solder the connection. 
I have no issues with the Millennium, just thought i might try the Cresendo. My hunch it is superior to the Millennium.