I would have to agree with Chainsaw Willie. Your current amp is probably well "voiced" for the theremin. Like a keyboard amp, it has a high frequency driver most guitar amps don't have. It also has built in reverb and chorus, which is a nice feature.
While the Roland Cube would work, I would not recommend it. The low frequecy driver is only 6 1/2" in diameter. This is likely too small to produce an appealing result. The bass will likely be thin and "buzzy". It's not that you need really deep, strong bass - rather you want a speaker large enough to give you nice "tone". Your Fender has an 8" speaker. Something in the 8" to 12" range will usually give you the best results.
Coalport sometimes uses a 15" speaker but without an enclosure, which reduces the boominess it would otherwise have. It is a driver (speaker) often used for lap and pedal steel guitars.