Back in the olden days when it was called MassDrop, the famed ESP/95X electrostatic headphone and included energizer (amplifier) was released at $500 and made it the cheapest electrostat/energizer combo you could get anywhere.
Koss has a no-questions-asked unlimited transferrable warranty on all of their products so as long as they're in business (and you can get a hold of them) these headphones can be repaired. IMO this is the most honorable warranty of any audio company I have ever interacted with, it's insane.

I'm selling the whole shabang with everything that was included when it shipped to me in 2018. I can't hear anything wrong with them, and fed thru the Modi+ they sound real nice. I'm just getting more into portable gear cause realistically most of my listening is done on the go or away from home so audios 'lectrostats!

Shipping anywhere to USA is included with the price, internationally I'll pay 50% shipping

Due to inflation these will be going for $695