I haven't used the SRD/7 so can't compare. The Lambda headphones can plug into the SRM-1 Mk-2. I have found this unit works great. There are two earspeaker jacks, one "Normal" and one "Pro". The "Pro" jack is for Pro or Signature earspeakers. According to http://www.stax.co.jp/ENG/History.html, the SRD/7 came out in 1971 and the SRM-1 Mk-2 in 1982. Can't speak to original (large) price difference but it should be possible to get an SRM-1 Mk-2 for somewhat over $200. These appear both here and on Ebay from time to time.
Stax Hdphns: SRM-1/SRX versusLambda Pro/SRD/7
I am trying to find out which Stax system was better in terms of the headphone amp or the transformer box--I have the Lambda headphones with SRD/7 transformer "box" that can switch between speakers and phones. Headphones sound very good but also a little "hard"; and I have concluded it is the fault of the SRD/7 box. Therefore, is the SRM-1Mk-2 headphone "amp" an upgrade from the SRD/7 transformer box?? Can the Lambda phones plug into the SRM-1?? What was the price difference of these two systems?? and the year they hit the market. Thank you, SJ