Installing Plywood Subfloor Over Original Plank Subfloor

I purchased an older home and plan to replace carpet in the bedrooms with engineered hardwood. After removing the carpet, there is a 2.5" plank board subfloor; fairly good condition. I plan to screw any loose boards directly into the joists and then lay 5/8 t&g plywood over the planks, screwing the plywood to the planks (not the joists to prevent slip). Is this the best approach? A few other questions:

- Does it matter which direction I lay the plywood? Perpendicular to the planks or the joists? I eventually intend to lay the engineered hardwood parallel to the planks.
- Would it help to lay 15lb felt between plywood and planks?
-If there are dips in the planks, what is the best way to level the plywood? Shims under the plywood?

Thanks for your help.

 

Put the plywood down perpendicular to the joists.

 

Thanks. I just want to confirm that it will be ok that all levels of floor are going in the same direction (original plank floor, plywood subfloor, new engineered hardwood).

Also, there are some dips in my original floor. Would you recommend I put some asphalt shingles between original floor and plywood to help level?