I had the SRX-MKIII with the SRD-7 for several years. Of all the 1970's headphones I had, the Stax was the only one that would sound good against today's better headphones. A genuine Stax amp probably won't be cheap. I haven't researched these either, but I got the impression that today's Stax headphones do their amping differently than in the 1970's, when we had to connect the SRD-7 box to the speaker terminals of a power amp.
The Stax SRX headphone (and maybe other Stax headphones too...) was the first headphone I bought that contained a freq. response curve that wasn't flat or nearly so, intentionally. The Koss ESP-9 and all of the Senn's from the 414 to 424 and up came with response curves that looked exactly like the headphone sounded, but the Stax high freq. curve was very choppy, and Stax's literature had some incomprehensible verbiage that "explained" why. And to top that off, the reviewers at that time had the opinion that the SRX-MKIII was a little on the bright side.