The SRD-P and SRD-X are portable amps for Pro and Normal bias Stax, respectively. I have one of each. They are heavy, since for portable use they require eight C-Cell batteries. You can alternately use an AC adaptor.
Here's some pics of the SRD-P (My SRD-X is at the office).
Drats, I forgot to include something to indicate scale. The unit is about the size of three paperback books stacked together.
Portable Stax amps are somewhat rare. You may have a difficult time putting your hands on one.
I was thinking of driving regular over-the-ear earspeakers. Forgot about the In-Ear SR-002＋SRM-002. Those are battery operated for portable use, too, and readily available on eBay.