Paul,
There is a website for Stax:
www.stax.co.jp
which has a history section that shows old products.
That is the main thing, that these are older models. The new driver units seem to be small amplifiers themselves. I am not expert enough to know which old model might be best or how you might find one. They are driver units for their own headphones. The SRD series seems to be the driver unit that hooks up to your own amplifier. I will confirm my model tonight, but I believe it is the SRD-7. There are a couple of listings here on Audiogon.
Good Luck,
Bill