Hi, Michael. GordonC here. [i]Very[/i] experimental thereminist. Proof: http://myspace.com/beatfrequencyuk (Check the videos page.)
From what I have seen and heard in the video and audio clips on the theremaniacs website, their instrument appears to be a reasonable one stick theremin.
One stick theremins are by no means ideal for melodic playing, as one might imagine volume is an important component of music. (Perhaps not as much as pitch - a fixed pitch, variable volume theremin would be as much use as a poke in the eye - but nonetheless.) There are alternatives to a volume antenna - the cheapest and possibly most useful is to know that sudden, rapid movements in the pitch field create harmonics in the audio output of the instrument (via FM synthesis - when you wave your hand in the pitch field, it really is a wave in the scientific sense) which spreads the energy over a wider range of the audio spectrum, causing a reduction in the perceived volume. In other words when you have finished playing a phrase snap your arm quickly out of the playing field to avoid playing a long glissando on the way.
Also you may like to know that the first version of the theremin developed by Lev Termen - the aetherphon - was a pitch only instrument, which was equipped with a cut-off switch for staccato playing, and an expression pedal for, er, expression. Not as good as a volume antenna but if you've got a pedal knocking about it's worth a try. Likewise, if you can wire a push-to-break switch into an audio cable, I'd say give it a go - if you make it hand-held you have a close relative of the aetherphon, the electronde. Scroll down this page for a sample of a simple recreation made with a modern theremin.
http://peterpringle.com/ondes2.html
Yes, the range is adjustable, and from the video clip looks to be a sensible size. What I cannot tell is the linearity of the instrument. The various notes of the scale are not evenly spread apart in the playing field like a piano, the low notes are further part, the high ones close together. In between the two extremes is an area where notes are comfortably spaced and don't vary dramatically from one end to the other. The number of octaves that lie within the playable area varies from theremin to theremin - in theremin-speak, the wider the playable range and the more evenly spaced the notes within it, the better the linearity of the instrument - my guess is that the theremaniacs instrument does not have great linearity, as this is usually a feature of high-end instruments.
The B3 is not an easy instrument to play, simply because of its size. The playable pitch area of a two stick antenna can't extend over the volume loop, and the pitch rod is rather close to it. The video is valid, but Mr Grillo has invested a phenomenal amount of time in acquiring the skills to play it so well. Consider that a trained classical pianist can make a child's toy piano sound better than it ought to.
I would say to either the B3 or the theremaniacs, have fun, don't expect miracles - these are low end instruments, and if you can wrangle a decent tune out of either then you probably have something of an aptitude for the instrument and may want to consider getting, say, a moog etherwave if it's going to be an important part of your live performance.
Oh, and bravo for wanting to play melodic theremin, but if you're doing space rock, nothing does space effects like a theremin, and there's absolutely nothing wrong with using it for both melody and effects! You have a delay pedal lying around? Good. Have some fun - you deserve it. :-)