I bought my 2SW some 20-25 years ago together with the matching subs. In my opinion the original subs are as bad as the side speakers are good.
I have tried two sub systems. The first was a home built arrangement with a high quality 15" woofer in a closed box per side. The speakers were electroniclly forced to go very deep. Although good it was not the answer. Then I managed to lay my hands on a system that was a copy of the sub system that came with the famous Infinity IRS. Mine are not the originals but uses the same electronic circuit and the same woofer elements. The originals caries six 12" woofers per side and mine five. These are arranged in a top to bottom array as a line source which is precisely what the Bevs are. Google for the IRS to see the arrangement. The sound from a line source decays with 3 db for every doubling of the distance as opposed to a point source (like a box speaker) which decays at a rate of 6 db. To have a sub systems that mimics the side systems decay characteristics must have some advantages I believe. I have tried to match sub systems to other electrostatic speaker that I have owned and never managed to get them to blend in in a natural way to the side systems. In particular the problem has been in dynamic passages.
These units were exactly what I was looking fore. They were placed some three feet further back than the side systems. In this position they blended very well. The first impression was of much better dynamics, the second impression was of a very airy sound stage, the third impression was of a much much improved low register. It was also evident that there was a good match in dynamic passages.
There are so many new systems out there that I have not tried but my advice is that you really try out the candidates at home. I think that there are very few systems that will meet your requirements. If you can lay you hand on a IRS sub system I think that you are home. There is a lot more to say about how this system operates but I believe that I am running out space on this
post.
