I got a pair of Lambda Signature Pro's and an SRM-T1 for about $1,000 on Audiogon. Love them. I put new earpads on the Lambdas.
I also have a pair of ESP-950's. They are also excellent, and less costly. The ESP-950's are definitely a high end product in terms of sound, and do give you the "electrostatic" experience- but they do not sound like Stax. Are the Koss better or worse? I would say, no, they are as good in sound quality as the various Lambdas out there, maybe even close to the SR-007's in some ways. But the Koss are DIFFERENT sounding. A different voice. If I had to characterize the Koss ESP 950 sound I would say, more bass impact and a somewhat more prominent midrange than the Stax. Good, smooth treble from the Koss, but not the same kind of 'transparent balance' that the Stax have. I like the Koss a lot, but they are different from the Stax. Only Stax sound like Stax, though Koss and Stax both offer that very low distortion, fast-transient-clarity electrostatic sound. And even though the Koss headphones use a lot of plastic- the Koss amp is mostly plastic, too- the Koss are not cheap crappy plastic headphones. For plastic, the headphones themselves are well made and fit and finish are quite good, and the plastic Koss amp hits well above it's weight- pretty good sound considering the low price of Koss amp. But in the end, only Stax are Stax.
If you can, listen to both side by side and decide which you like best. In my case, I LOVE having BOTH!!!!!! I like to switch off. I'm lucky that I can have such things.