Which Stax headphones- best bang for the buck

I would like to replace my Koss ESP 950 headphones with Stax headphones. Naturally, I would like to make a step up. Which Stax headphones (and amplifier) offer the best sound for the money and why? Thanks in advance.

I have a Stax Classic 3030,and I am very happy with it!Class A amp,and same electrostatic element as the Signature 404.
I am a former owner of a pair of Stax headphones, and agree they make fine products. I recently switched from my Stax SR-80's to the Sennheiser HD600, and think they are a substantial improvement over the Stax. Infact, I said in a post earlier this week that I think the HD600's are the best headphones I've heard, other than the Stax Lambdas which were a lot more expensive. I urge you to give the HD600's a good audition before you make a purchase -- they can be purchased new from AudioAdvisor right now for $300, and are often available on Audiogon for around $225. Just a suggestion...
You didn't say whether you were going to buy new or used. I have the older Lambda Professional and Lambda Signature earspeakers and SRM-1/MK-2 and SRM-T1 headphone amps. I recommend the Signatures (or equivalent in the current Stax line) because the bass response is far superior to the Professionals. The amps are in different systems, and I have never done an a/b comparison. Both sound good.

I haven't heard the Koss headphones. I agree with Sdcampbell that the Sennheiser HD600's are excellent for the price, but the Signatures are better.
I had the Lambda/Class A amp combo and thought it was great. I also used the headphone amp as a preamp for my regular system since it was also designed for that purpose. I eventually got out of headphones because I preferred to feel the music throughout my entire body as well as just my ears. Even though I don't play very loud, there is something about the feel of the music that is not experienced when you wear headphones.
Koss ESP 950 were state of the art in yesteryear. Unfortunately, they can't begin to compete with Stax models available in 2016. My choices would be the SR 009, or Omega Mk1. It's always a good idea to pair them with a Stax driver unit. There are many headphone amplifiers that can outperform even Stax amps, like Blue Hawaii or Woo Audi, but both are very expensive. Other brands to consider would be Audeze, Hifiman or Oppo.   
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How about the Jaeklin Floats?