From: Tim Goetze <tim(a)quitte.de>
  m = l + r;
  l = dry * l + wet * reverb_l (m);
  r = dry * r + wet * reverb_r (m); 
[ ... ]
 'True stereo' is, of course, marketing BS. You
may argue that I suffer
from over-exposition to commercial environments; I concur. :) 
Dattorro's plate reverb of Griesinger type also feeds the M = L+R
signal to the main reverberator. However, that may not be a good
idea if L and R will sound bad when summed. When summing is done
that way, only the bad signal is going on inside the reverb. Nothing
turns it good.
Juhana
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