I don't have a Burns theremin, so my thoughts should be taken with that in mind.
The advice I was given when I started was to get an amp designed for an acoustic instrument, and that has served me well.
I was very pleased with my Fender Frontman 15R and it was plenty until my ambitions grew (which took about a year.) Now I have an SR Technologies Jam 150 Plus and it sounds great to my ears.
I agree with AlKhwarizmi that reverb is good with theremins.
I suggest not rushing to buy a new amp. A lot of the sound of the theremin depends on the skill of the player - once you have acquired some skills, then take your instrument to a local music shop and try out a few amps and see which you like.
Not sure what you mean by a PA - in my mind a public address system is a thing with lots of megaphone style speakers that is not well suited to music. Wikipedia defines it as "is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a sound source, e.g., a person giving a speech, a DJ playing prerecorded music, and distributing the sound throughout a venue or building."
If you mean "a [i]self-contained[/i] electronic amplification system with a mixer, amplifier and loudspeakers" then I would call that a "combi" - my Jam 150 Plus is a combi - it is occasionally handy to be able to plug a mic and/or another instrument at the same time.
(Also, not knowing the Burns instruments or your amp, it is possible that the signal level from the theremin is simply too strong for your amp and the sound is being clipped.)