This may sound stupid.  I have not followed the entire discussion so I may
be repeating something, or I may be naive about audiophiles, but did you
consider sending the audio signal through an op amp with a log gain
network?  As far as matching is concerned, that could be achieved by
trimming or perhaps muxing the same op amp between both channels.  I
believe that Burr Brown has some very fast settling time audio amps which
might work.  But it has been a while since I saw the specs and you would
have to review the BB data books.  If the settling time is appreciably
greater than the required response, it may be possible to use the same
device which would solve the matching problem.  Just an off-the-cuff
comment.