I don't know why but ficus tend to be the tree of choice for audiophiles- do a search at the aa and you'll find tons of people are using them- in particular with planers/electrostats.
I have a concrete wall(block) behind mine with paneling over it and have NO problems and in fact have the fastest response(at all freq. ranges) I have ever heard from any system. I don't think you could go wrong, just use some rear diffusers or Ficus trees in the corners/front wall(behind the speakers) and you should be fine.
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