I live in a 100 plus year old condo building on top 3rd floor in Chicago that has steam radiator heat. I am replacing the original ¾ x 2 ¼ solid oak floors with ¾ x 3 ¼ solid oak floor. I have a pneumatic cleat nailer. After ripping the old floor up I discovered the subfloor is 1x8 planks ran perpendicular to the floor joist with ¼ to ½ inch gaps in between the planks. The old floor was also ran perpendicular to floor joist and parallel with the subfloor planks. I’m considering laying ½ plywood on top of the planks because I want to install boarders in a room so I will be running some flooring parallel with the joists. To reduce noise in between units I was also planning on installing QuietWalk Plus underlayment. Due to conflicting advice I’ve come across I have some questions. After securing planks, should I screw plywood into joists or only into planks? Would I be better off installing QuietWalk underlayment between planks and plywood or between plywood and oak flooring? Link to QuietWalk Plus: https://www.mpglobalproducts.com/floori ... walk-plus/ . For the room with the boarder, installing the center of the room first, then using straight edge to cut floor 1 foot from edge, route a groove in floor, install a spline and then finish the boarder along the outside edges seems the easiest way to install a boarder? Thanks in advance for any advice!

I would lay the plywood over the planks. I make sure there are three fasteners in each plank where it crosses the joists using an MIII stapler with the T&G part removed from the base, using 2 inch staples, with a nailing schedule of 6" on perimeter and 8" in field of each sheet. Quiet walk is next, stapled in place with little staples.
I like to lay the border first then cut in the field. There are little details that I like to get worked on as the border is installed. I like to have better choice of the best boards for the border, too, so I open bundles in the center of the room. After the border is laid I start at both edges of the field and meeting the middle with a rip, or sometimes two rips if the rip is just over one board width or tapers so I don't have a taper that ends in a sliver. I use sub-floor adhesive where the field meets the border. You can glue the Quiet walk down there and glue to it.
Have you thought of using a feature strip? Walnut looks good with a red oak floor and red mahogany looks great with a white oak floor. Staining the floor will enhance the grain pattern of the oak, but not change how well the feature strip adds to the architecture. The feature strip highlights the shape of the room. I like to make an 18" or wider border so the furniture against the wall doesn't hide the border.