nieradka, if you just want to play the theremin as a bass, why don't you learn the bass? It is much easier to play than a theremin, far more accurate in pitch, and in its home (bass) register it sounds much better than a theremin.
Back in the mid 90's, when all we had on the internet was the "alt.theremin" message board, there was a woman who came on line saying that she wanted to learn the theremin for one reason only - so she could play the Bach unaccompanied cello suites.
I suggested to the lady that she abandon the theremin and learn the cello. First, because it is so much easier to play than the theremin, and second because you can't play the cello suites on a theremin no matter how good you are!
People who are unfamiliar with the limitations of the theremin often get ideas in their heads about what they would like to play. They rarely listen to the advice of others who have "been there and done that", and have to go through the disappointing process of discovering the shortcomings of the instrument first hand.
I believe unrealistic expectations about what is possible on the theremin are responsible for the very high drop-out rate among theremin newbies. (I'm not saying this applies to you, I'm sure it doesn't, but it is something I have observed frequently in the past).