Thank you..."can be typical" is the answer I was hoping for. There's still the possibility that the floor is not completely flat, but just knowing it might give a little even if it were flat is reassuring.

Originally they were just going to tighten the plywood but then cement board was suggested so we went with that. We thought it might be sturdier and that replacing the entire sub-floor would be one way to ensure they got all the squeaks. (Which they did...not a peep now.)

Concrete board? As in Durock cement board? That is used as backing for tile. IT IS NOT STRUCTURAL. Any good flooring pro would recommend screwing down the existing plywood (unless there are serious issues). If the existing floor is marginal (too much span for the plywood) they may also recommend adding another layer of plywood to further stiffen the floor.

If the plywood was removed and something like durock installed you should seriously have a structural engineer look at it right away. Personally, I wouldn't walk on it. I value my life too much and cement board plus some engineered wood is not a safe floor.

^^^^^^ this. I can't believe they used cement board.

Correction...they put the cement board OVER the plywood. They did not replace plywood with cement board. (Whew.) Sorry, I misunderstood but have since received clarification.

Thank you all for your comments, though, it's been very helpful.

That entire procedure is ok, not what I would have recommended, but nothing unusual there.

I'm not a floating floor fan, mostly because of the sponginess(due to the flooring and the foam along with not making sure the subfloor/concrete board was level) and the hollow sound is the result.

I do understand the appeal of that type of flooring, as changing it is much easier and less expensive---which will be an asset when the flooring deteriorates.

There are two things contributing to this issue: uneven subfloors (cement board is expensive and unnecessary for engineered hardwood install...WTH? but not "dangerous" - just useless) and an underpad that is probably a bit too spongy. Those two things put together will cause a floor to BOUNCE.

If they had properly taken care of the squeaks (easy to do but takes an hour or so of time) and then leveled the floor (easy to do but takes some time and self leveling compound and some sanding afterwards) you would have had MUCH less movement than what you are seeing now.

The question I have at this time is two-fold:

1) Is the subfloor "flat enough" to maintain manufacturer's warranty (all floors have a flatness requirement and they are written into the installation instructions)?

2) Is the under pad/underlay APPROVED for use with this floor? Many floors have limits on their underpad (for this VERY reason)?

You can find the answers to both question on the internet. Simply do a google search with the name of your floor and the work "installation". You will come up with a link to the DOCUMENT (ignore youtube...it doesn't show the LEGAL specifications).

And the only way to find out how FLAT the subfloor is....you guessed it...is to remove the planks and test for it (10ft straight edge...if in doubt use the ration 1/8" over 10 feet...it will get you close enough to most regulations that you should be OK with that measurement).