Hello Red!
When new Diva MR's are installed they are tensioned in manufacture, and weight tensioned to relieve that tension in an even and controlled way. As you play (normal use) the ribbons release a bit more tension, then over the next few years (normal use) the MR ribbons relax a bit more , ongoing until they eventually drift out of ideal specifications. This process takes 7-8 years, by which time it's time for a new pair. If they have had more use, or abusive play levels the life estimate can be dramatically shorter...
The Diva MR and Duetta MRTW are related, but have different specifications, and operate over different ranges.
Tension check - to attempt this involves destroying the ribbon.
You have some rare and amazing speakers there, be kind to them
Take care - Graz