I presumed from your original post that In your case you will not need to worry about underfloor heating if the boards are on joists,Gluing a engineered board to a plywood as you said was the architects spec is one job,any other way cost more time and money.You can lay solid boards straight on the joists without ply if you wish,but it sounds to me like the ply is all ready laid,so its best to chose a few possibility's and ask your fitter for the pro,s and cons of each.It is also worth ,as the previous poster touched on is to check that the timber comes from managed sustainable forests.