I'll add my voice to those saying plywood and firm cushions. Most sofas are
far softer than most people have any need for -- it's sort of a contest for
who can provide the most travel in the springs and cushions regardless of
whether it's useful. My favorite sofas are made to begin with using solid
foam cushions over wood slats or solid wood. My butt provides ample
additional padding if needed.

Edward
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Art Works by Melynda Reid: http://paleo.org