Finally a subject I understand!
Almost every 15 watt modern amp (and quite a few vintage ones) I've seen go from crap to something I won't say in the company of ladies. Their biggest shortcoming has been in the cruddy 8 inch speaker, small cabinet, and ill conceived tone controls.
The only reasonable one I've seen was a Vox pathfinder, and it was modified with a 10 inch speaker and tone stack tweaks.
A decent amplifier will cost as much or more than your theremin did.
One could do much worse than Fender. I have a vintage Fender deluxe reverb amp that sounds wonderful with a theremin, but even a modern, re issue of it will set you back about 800 US dollars. Mine has been upgraded to 40 watts, and has two 10 inch speakers.
Guitar amps will give mixed results because they are designed around an electric guitar's response. They will have a midrange notch around 700 hz, varying with different models, and will overdrive at theremin signal levels.
In my sometimes not so humble opinion, a small, 100 watt class PA amp with a 2 way speaker or two should serve well. The bigger the speaker, the better your low end will sound. There are quite a few reasonably priced ones with 15 inch speakers for the lows. I would not go any smaller than a 12 inch. A powered monitor speaker is also an option. If you have a decent home stereo, that would work in a pinch.
That being said, the amp that most often has my theremin plugged into it is an all tube 30 watt single 12 inch speaker guitar amp that I designed and built.
An amp designed for bass guitar would probably work well too. I have an all tube, 200 watt Fender bass amp with a 15 inch speaker that I'm going to try soon. It's in storage now, and because it weighs about as much as a small nation, I've been putting off dragging it home.