Problems in choosing the right amp to play on the streets

Hello thereminists! Need your advise on picking the right amp for the street performances. I discovered this is not an easy task. I have been considering: 1. Weight, 2. Battery powered 3. Watts 4. Size. After all, I decided to go to the music store and checked Roland Cube Street EX and Roland AC-33. I ran my Subscope through each amp.
At first, I tested them using power source (220 volt) Both worked very well and theremin sounded good ( Roland AC-33 sounded more alive). And then, when I tested them powered from batteries, they sounded horrible, lots of distortion. So, I guess it has to do with phases and circuity? How to solve this problem? Thank you in advance!

Hi

I'm based in London and looking for a theremin player to record some brief parts for a song i'm doing. Are you interested in having a go? We can correspond a bit and ill answer any questions you might have. Thanks   

If a musician wants to use the theremin amplifier for battery-powered, the first thing you need to think about grounding equipment.