I am preparing to install some unfinished 3/4" Brazillian Cherry strip flooring (something I have never done). Currently, the floor consists of 5/8" plywood subfloor with 5/8" Particle board underlayment (decking) on top. My question is, after I remove the particle board, would it be better to put down 1/4 to 3/8" plywood underlayment with 15# felt between each layer to sturdy up the floor or should I just put the hardwood on top of the 5/8" subfloor with the felt? Thanks for any suggestions.

NOFMA states that the 5/8" plywood subflooring is the minimum acceptable subfloor for 3/4" nail down flooring, as long as the joists/trusses are placed no greater than 16" oc. You must lay the flooring at right angles (90 degrees) to the joists to be able to properly support the flooring. If you wish to run the flooring parallel with the joist direction, you can either add blocking between the existing joists or add 1/2" CDX plywood underlayment.

Thanks Gary, I am not necessarily trying to change the direction of the floor, but looking more for stability.

By installing the 3/4" b. cherry floors across the joists, you are adding more stability. If you lay the cherry floors parallel to the joists, the floors will sag, creak and possibly separate more easily. If you want to add more plywood for maximun stiffness, then go ahead, no harm in it.

Can you really substantiate this...have you actually seen this happen? I mean with 23/32" Sturdifloor T&G w/16" OC engineered i-beams, is the floor really going to sag with 3/4" T&G nailed down hardwood floors? Truely is this really going to happen or is this warranty voiding fluff? Come on...has anyone ran parallel in this situation and had problems?

It's going to sag regardless. With the boards perpendicular to the joist though, the sagging isn't stressing the joints. Parallel to the joists and the flex that is inevitable is going to be stressing the intersection between the boards.