I have standard subfloor + 1/2" plywood on top, to correct for joists going different directions, so that all hardwood boards can be laid one direction for a clean look and avoid sagging where they would go parallel to the joists. The sagging problem apparently solved - but would it still make sense to go through the expense of quality underlayment? Or the extra layer of plywood be just as adequate for sound/vapor/heat control? Thanks
I would. I put underlayment for all engineered. I spend about .25 a sq/ft for my high quality rolls so not sure what you are looking at. The point of the underlayment for engineered flooring is to provide a surface for the wood to slide across (wood on wood gets stuck sometimes) and to decrease the noise between hardwood and subfloor.
Imagining your situation I feel like you might hear Clydesdale clicking and clacking if you don't install anything.
I believe the point of underlayment in your situation will be controlling moisture movement from below. That is, you want a vapor barrier under the floor. Now, the underlayment can achieve other things as well, such as providing a "loose" surface for a floating floor (as discussed by @DMoore), or enhance sound deadening.
I read your post and comments, and I'm still not 100% whether you intend to float the floor, or nail/staple it down. I have installed wide engineered hardwood both ways, and I far prefer nail install where feasible. Mostly that is because gluing the tongue/groove is way more time consuming (assuming your engineered is not click lock or the like) than nailing. But too, a nailed floor feels sooo much better than floating. But I digress....
Because this is a first floor, I assume sound deadening is not a requirement. So, for a floating floor, I would use a 2-in-1 underlayment like the Robert's product here: http://thd.co/1AGfcoZ. For a nail or staple down, use Fortifiber Aquabar B (http://www.fortifiber.com/aquabar_b.html). Either product will do the trick....