I would agree with your review. I never have had any squealing with mine over the last 20 plus years. They were and still are the bargain to seek after in electrostatic headphones.

I purchased mine second hand. They are as new, and i have never had any problems with them.

Good review and pretty much right on target.  I am particularly fond of electrostatics and have had the ESP950 for about six years.   They are very neutral and natural with virtually no coloration.  Light and comfortable the plastic construction is not a drawback to me, they manage to still look pretty good.  I have had no issues with mine other than when it intermittently sounded slightly off in clean detail on one channel.  I sent them to Koss (it was only $5 then if I remember correctly).  They gave them a complete check, found nothing wrong and put on a brand new set of earpads even though mine were not in bad shape.  That was several years ago and all has been well.  I had some head fiers request I bring them a couple of times to meets and they always received a reception.  I even have an ESP 9 and ESP 6.  Both are head crushers and totally too heavy and uncomfortable but the collector in me still enjoys the history.  As I was in the retail audio business since 1972 I was an authorized dealer for Stax, AKG, Sennheiser and Grado also.  Stax is my main interest in electrostatics and I have seven in my collection but as stated the Koss is truly worth a listen and can be quite the good deal.  Thanks for the good review.

Koss Owner's Manual:  https://www.koss.com/media/productfileupload/ESP950.pdf
 
http://www.head-fi.org/image/id/8869947/width/100/height/100/flags/ZC
 

Wow, it's not hard to see these as being a great value for $290. They're only available new via Amazon-affiliated distributors for around $1000 now. I believe at the 1k price point, one would have other options for similar sound, including Stax, but I could be wrong. I would love to compare these to the AKG K501.