This question was asked previously. The above responses touch on the reason why many give before realizing what these speakers are capable of. Tonally I have not heard a better speaker than the VSM, they are just superb in this regard. Add to this their extreme resolution SOTA in my experience and immediacy and you have a transducer that prioritizes system matching to realize their capabilities.
While I feel they work well with a wide variety of tube gear, they are worthy of the best one can afford. Best in my experience with this speaker are the Joule and Berning as Jim notes above. Equally important as the amp interface is the wiring. They will highlight a synergy problem in this regard so it is very important to get it right. A bad choice can lead to less than stellar results and frustration. Bobby P is the best resource for the ones that work with his speakers. Silver cables in my experience are a no no with the VSM contrary to what I have read from others.
The main things to consider with the VSM are amp interface, wiring and room size. I am not sure they are the best choice in a very large room. While they can be played loud they are MUCH better at more moderate levels. They WILL play loud and their dynamic capabilities are quite extraordinary but lets face it, they can't compete with a large system in a large room in this regard without sounding pressed. A subwoofer might help in this regard but I can't imagine one that wouldn't compromise the electrostatic coherency that this design is capable of. These speakers are all about refinement and resovlving the musical information present on the recording. Bobby P is working on a sub for folks that want more bass but in my medium sized L shaped listening room, I feel no lack in this area, it just won't have the power of a larger system. The VSM does orchestral and large scale choral quite well with a wide and layered presentation of the venue.
The last and most important thing to consider is the presentation. The Merlins are about as neutral in this regard as any speaker I have heard. A close miked recording might not sound as appealing on the VSM's as a speaker that is voiced to pull back the sound and they may sound "bright" to some listeners but to me they present the music as it was recorded. Some won't like this but I feel that the design is near perfect in this regard. Honesty sometimes comes with a price but on balance it is well worth it. As you might guess I love the speakers but also recognize the fact that some listeners will feel they are maybe too fussy and give up. If you choice them and stay committed after the purchase and have a small to medium size room, you will be rewarded with a musical package that is tough to beat at any price.