On Sun, 16 Apr 2006 14:57:09 -0400, Davoud wrote:

Is it necessary or desirable to use a soft underlayment when installing
3/4" nail-down hardwood flooring on 3/4" plywood sub-flooring?

No. It is necessary to be sure the plywood is good quality and nailed
firmly to prevent squeaks. My old home has a hardwood rough plank
base and then harwood flooring nailed onto that. Jumping up and down
on it is like jumping up and down in the paved driveway ...solid.