Hello vero,
"Out of curiosity, if an amp needs to be grounded in order to work with a theremin, are battery-powered amps able to be used with a theremin?"
That is a very good question and I would say the answer is yes, but only if your theremin power cord is a three prong plug. All of mine are. A theremin needs a good earth ground to respond properly and to not distort the sound, there is a little more to it but that is the basic idea.
People that play on street corners "in my opinion" need to drive a long metal rod into a flower bed for the best results and leave it there for the next time they return to play. I have heard of people standing on conductive mats or the audio cable to the amp for better response but the sound will still get a little weird.
Christopher