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
I don't know the price difference, but I have the Lambda pro hooked into the SRM-1/MK-2. How does it sound? Hard. These headphones are not forgiving at all. I have heard that their tube amps are far less "hard" and have been interested in trying a pair for a while. I can see why these headphones are often used by recording engineers--they tell it like it is--with no politeness.
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, 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.
The best way I've found to run a pair of Lambdas is to use the Class A headphone preamp Stax made for them in the 1980s. It was called the SRA-12S. I had one, and used it for my regular preamp, and the Lambdas. It had output jacks for 2 sets of Stax phones. No attaching to the amp's speaker leads was required, it was all done in the preamp. Let me tell you, this thing got hot! You could fry an egg on top of it. There were inputs for 3 types of phono inputs, MC,MM,and Stax proprietary strain gage cartridge. There was a tape loop, aux, and at least 1 pair of main outs. All volume, balance, and switching was done on the front and the 2 jacks for ES phones were on the front. I ran this as the main preamp with a Stax DA-80 amp for awhile, and it sounded pretty good. Between the 2 space heaters, it was too hot for me to keep. I just knew that all that heat was no good for the passive components in those units. But the ability to properly drive the Stax phones was unsurpassed. It was built like a tank, with all the quality you'd expect from the old Stax line.