I've built the PaIA kit. Its a good kit, although I supplied my own antennas and enclosure. Moog will sell you their antennas and fittings for not too much money, and these are adaptable to the PaIA kit quite easily. Without a doubt, the PaIA kit, the "ThereMax" is the most moddable, and most adaptable out there, as its fully proven and a lot of input from users has been gathered up and tested by the masses, weeding out the BS mods.
A word of caution though - PaIA's founder, John Simonton passed away a few years back. John was an incredible tinkerer, and a tinkerer with a degree (several I believe) in electronics design. I met him, in high school, back in the early 70's when the schools guitar teacher had him in for a presentation on guitars and synths. He had a whole board full of stuff rolling on a little wagon (I think he was in NY for a trade show). I remember with a ring modulator and some sort of delay and some sort of reinsertion and a pedal, he was making a Fender Mustang sound quite human in vocal quality - with no talk box! They may not be offering things for a whole lot longer, although from the website, it appears that they're doing ok still, with new items being added. John was less a business man than a creative genius, but the new GM of his company is more business inclined, and hopefully the kits will be available for some time to come. --- I'm saying if you want one, get it now.
Interestingly, my wife babysat for Bob Moog's kids when he had his outfit outside of Buffalo NY. That was all before I met her, and when she was in college. I've played the Moog (or played with... does one ever truely "play" the theremin?). The sound quality, out of the box is superior to the PaIA. It just sounds fuller. There are some harmonics going on with it, I'm sure, whereas the PaIA is more of a pure waveform.
In reality, the theremin should be a pure sine (or sine-like) wave output, with the only deviation from, or addition to that, being had in the audio stages later. This is because the theremin is really only a "human BFO" where the lower set of generated frequencies is in the audio range, while the upper set is quite ultrasonic and filtered out of the audio path. In practice, the thermionic theremins (tubes that is) did not have great audio or great oscillators either. The result was a very violin toned, almost vocal quality to the music... er if you call it music. Bob Moog put his inventive skills toward re-creating that richer, violin/vocal tone in his instrument.
In either case, the addition of pedals in the signal chain is a good thing - some little bit of overdrive, some filtering, some auto-wah, etc... you get the idea.