My used Magnepan 1.6's are missing sparkly treble. Why?


I recently bought a pair of used Magnepan 1.6 speakers.  Unclear how old they are but they appear to be in good condition and the panels look good when shining a flashlight through.  And the speakers sound good except they are almost completely lacking in that bright sparkly treble that I thought they were known for.  Something like  chimes or a triangle simply isn't in the music played through the 1.6's.  If you stand right next to the speaker you can hear those instruments very muted but not at any normal listening distance.  I've tried googling to see if this is a known problem with these old 1.6's but without any luck.  

Does anybody know what might be causing this or if it is fixable?
pinwa
I'm not sure if this is simply confirmation bias or what but I did find  several different demagnetizing tracks on YouTube and after playing them I think the treble response is improved.  It is still somewhat muted but better than it was.  I don't think the problem with the speakers has anything to do with the removal of the fuse or a tear in the ribbons either of which ought to pretty much eliminate the high end response and that isn't what is going on.  I found a couple of 20Hz to 20KHz audio sweep on YouTube  https://www.youtube.com/watch?v=H-iCZElJ8m0 .  I stopped hearing anything around 11KHz which was also my limit listening with my Ananda headphones and that might be about all my 62 year old ears can hear anyhow.  So the 1.6's are responsive to at least that level.  I think the high end is just softer/quieter than it should be.  Maybe time to look into an equalizer?
most of the demag/burn-in disks can be had used on ebay or other sources.  There is a clarity of sound after using them that does fade with time.  
Seems a bit telling that you can't hear above 11k with your headphones too?  You may want to get your hearing tested and get that equalizer as well.  Maybe the hearing test will help you set up the equalizer?  Or go to a stereo store and see if you can't hear higher treble on other speakers. 
11K is about it for me.  I'm curious how high you hear?  Assuming this tone generator is accurate you should be able to get a pretty precise measure of the limits of you hearing.

https://www.szynalski.com/tone-generator/
Well I can hear up to 14k maybe a bit higher according to the tone generator.  It is a pretty annoying tone at 14k and I would like to say I can hear it at 15k too but maybe not so much.