No responses on my last post so I will try again.

Installing 1/2 CDX over 3/4 T&G Ply for underlayment for 3/4" solid.

1.) Do not really want to glue it down. Big mess if ever there are problems. Want to use 16 Gage stapler as suggested by Gary, but was wondering if the 16 gage and 1/2" crown width is critical. What about nailing with ring shank? Any magic in the holding power of the 16 gage staple. Would 18 gage work if applied every 4", or not enough power to pull the two layers together?

2.) Level the subfloor with 30lb before or after installing the underlayment. My gut says before.

My experience has shown that 16 gauge staples are superior in holding power to ring shanked nails. However, ring shanks have been used for decades. And I'd rather see you use those as opposed to skinny little 1/4" 18 gauge staples, especially if you are flatening with # 30 felt first. And if you are laying CDX underlayment, yes, try flatening first with the 30 lb or shingles then add the underlayment. Easier and better, IMO.

Any reason not to screw it down? My house had the squeekiest floors I had ever heard. Everywhere I have put wood down, I have prepped the subfloor by putting a deck screw right next to every nail, and then go back and resink all of the nails that had worked their way out. Yes - it is a tedious PIA, but the floors are quiet now.

Yes Dave, screws would also be a fine option as well. And the best in holding power.