The older Stax energizer systems (for the SR-30 / SR-80 / SR-5 / SR-Lamdba) wasn't the most efficient in the world - they needed a decent amp, with a modest current, to get their bottom end moving. Not outrageous, but not minor either. For instance, the SRD-7 never really sounded great with Denon stuff from the same era, but got much better with HK, Belles (a bit dark, like the amps themselves), PSE, B&K, etc. They do acquire the sound of the driver amp, and you can indeed tell the difference.
I would recommend, depending upon the earspeaker in question, a SS amp of a least 35W. The bigger the earspeaker (the larger the driver surface) the more power it needed, it seemed. I, honestly, never tested the SRM-7 with tubes. If you have a very large voltage swing with those tubes then they just might work very well, but I can't say without putting my foot, and knee, in my mouth

You know, FWIW, the SR-30's were less efficent than the SR-80's, and from those (seemingly) early years the SR-5 mk2 seemed to be the least efficient of all. IIRC even less than the Lambdas plugged into the SRD-7. But that was a long time ago, and me mind is slipp.................ing
