Thierry recommended a Yamaha MS-101 and I have been quite happy with that, they can be found used at quite reasonable prices, although you have to watch out for ground loops.
If I had infinite space and money and were building a new theremin monitoring system i would be very curious about using an 80s Magnepan planar speaker, they're not from the same era but similarly "ahead of its time", analog, aesthetically consistent, etc. Maybe if half a stereo pair or a center channel from a 5.1 system showed up on ebay.
La Voix du Luthier is a modern company making gear designed for monitoring odd / vintage electrical instruments but it's quite expensive.