battery powered amp/speaker for busking

Can anyone suggest a suitable battery powered speaker for busking with theremin?  I have been searching and working on this project for year or more with nothing but frustrating failure in my endeavour. At home I run my B3 pro through a talking box (gives a breathy/ human sound) with Hartke 100W keyboard amp. The tone is pure/strong. I am able to power the theremin and talking box with separate battery packs. My last hurdle is a battery/self powered amp/speaker. I have tried all types of battery powered amps but the result is a hideous growl/scratch each time. I have experimented with various noise/interference reducing plugs and boards to no avail. I've tried Roland cube,(have since learned that a theremin will not work with a guitar amp. ) I have since tried Bose, LG. as well as trying to power my entire set-up including my Hartke with a portable  AC power box but no good.  I live in Perth Western Australia,there are no theremin retailer  and I took a chance buying the B3 online, but have been pleased with it. There seems to be no expert in Perth who is able to help me with my frustrating project of being self powered. Does anyone have suggestions?  Could it perhaps be an earthing issue or something like that to do with my B3? I am not an electrical whizz but have researched as much as I can online.  I would be most grateful for ideas/ suggestions. Cheers Thuggboy Zigg.