So you're getting a theremin. Congratulations. You're going to want to hear it.
The best advice is to take your theremin to a well stocked music shop and try out a whole bunch of amps, then buy the one you like.
Oh boy, people are going to hate you. Listening to a novice thereminist trying out amps is not so much fun. Better to wait until you have some skills, because how you play affects the way your theremin sounds. Really. Two thereminists with identical instruments sound very different to each other.
So here's a suggestion of what to do in the meantime. Buy a good pair of headphones and a good mixer.
Good is the keyword here. You really want to hear your instrument at its very best with all its harmonics in place.
And the good news is that they will still be useful when you have got your amp.
The reason you want a mixer is so that you can listen to the theremin and your accompaniment (as well as your pitch preview if you are getting a theremin with pitch preview.)
I have a Mackie 403-VL23. It's small enough to drop into a travel bag. Every review I found said it was just as good as the big mackie studio mixers soundwise, and more than sufficiently versatile for my simple needs. It cost me £85 in the UK. I believe they are substantially cheaper on the other side of the Atlantic.
My headphones are Sennheiser HD 25-1 IIs. Again, excellent reviews, and I spotted them being used on the TV by a BBC sound recordist on location. That was the clincher. Also they prevent sound from escaping very effectively. I can have them as loud as I want (fairly loud, but not so loud as to harm my ears) and not disturb someone watching TV in the same room. They are about £100 hereabouts currently.
I am certain there is room for improvement in the sound quality, there always is, but I'm also fairly confident that the cost/quality ratio is optimal - specifically this is about as good as it gets for less than the cost of your theremin.