You should also glue wide plank if you wish it to stay flat as it will often crown or cup depending on the grain structure of each board.

Although it is usually fairly straight forward to determine the cause of cupping occasionally determining the cause of the problem is difficult. The construction details of the house, environment in the space below the floor, installation practices, and climatic conditions can all be important. Cupping is usually an indication of a source of moisture on the bottom side of wood flooring. It is more commonly seen on the ground floor of a house than on the second floor. A first step might be to measure for a moisture gradient in the flooring and subfloor using a moisture meter with long pins. If the flooring was properly kiln dried to 6% it is unlikely that the drying is the cause of the problem. With your humidity range you would expect the wood moisture to be 7% to 9%. The acutal humidity range is sometimes larger than the range given by the humidistat. Rosin paper does little to retard moisture vapor movement. You could refer to the NWFA publication 'Problems Causes and Cures'. You might end up hiring a wood floor inspector. From what I have seen some inspectors are much better than others

"You should also glue wide plank if you wish it to stay flat as it will often crown or cup depending on the grain structure of each board."

And then you run the risk of it splitting.

Wood changes shape and size depending on moisture level.

There is no practical way to either stop the changes, or restrain the wood.

Brickeyee....excellent post...good job

My 35 years in the business tell me it is a far better risk to take than having to deal with the customers dashed expectations of a flat floor.
If the wide plank manufactures are serious about alleviating the issue then do as Carlisle does and kerf the bottoms enough to mitigate the problem.