Sheathing grade ply under hardwood
I installed the sub-floor this weekend. I ended up going with 3/4" CDX over 15 lb roofing felt and nailing it down using Ramjet powder actuated fasteners. In some places I had to use multiple fasteners to get a solid connection, but I feel pretty good about it. The instructions on the Ramjet say that the fastener must penetrate the slab at least 1". The fasteners come in 1 1/2" and 2" lengths, but not 1 3/4", so I had to choose either 1/4" too short or 1/4" too long. I had already tried drilling holes for TapCons and discovered that my slab is VERY hard, so I decided to go 1/4" short. I think it was a good decision, because, using the highest power charge for the Ramjet that I bought (a .22 cal.), over half of the fasteners did not drive down flush. I had to drive them the rest of the way with a hammer. I used 9 fasteners in a 4 x 4 panel unless something felt loose. In that case I added more until it all felt solid. I put another layer of 15 lb felt over that and am now starting on my layout.
I know this is a rather old-fashioned installation technique, but it is time-tested and, if something does go wrong ten years from now, I know that I can repair it.