You need to do a little more archeological digging still. You need to know the whole construction of the floor before you can decide how best to install something new. I'd take up that plywood piece in the kitchen and see what's under it. It's higher than the surrounding floor, so needs to go. You should also be able to see from there what subfloor is under the original kitchen hardwood. It's too bad the space under your entryway is finished, but I'd still do some exploring from below to see what's under there, and to figure out the fix to stop it from squeaking. Is it drywall below or a drop ceiling? I know it probably seems like a pain to tear out drywall, but if you cut out a clean patch, like 2' x the width of the joists, it'll be relatively simple to repair. Just make sure to cut it down the center of the joists so that there's room to screw the patch piece to it later. Cut the hole under the area that squeaks the most, if you can. Have someone walk on and watch the floor from below to see what's moving. It might be as simple as pounding in a couple shims from below, or face-nailing a loose board from above. You want to find out what's underneath the hardwood. It might be laid directly on the floor joists, or there might be 10" wide boards that the hardwood is nailed to. Another thought is to take out a board in the hallway, since that's not original and you want to replace it. See what the subfloor is there. I am a bit puzzled at why, when you've stripped the kitchen down to the original hardwood, it's still higher than the hallway floor. Is your current surface perhaps not the original kitchen hardwood? Peeling back the plywood area will help determine that. BTW, I haven't heard of that stop squeaking product you linked to, so can't offer any personal opinion. And don't pour self-leveling compound over top of hardwood. It needs to go over plywood.

Do some excavation to see what is actually under the top layer--you may find several layers of flooring such as vinyl, linoleum, plywood or even hardwood if you are lucky. Once you get to the last layer, you may find your floors level. It is odd that the only subfloor is in that room--mine don't have any at all, just the very thick tongue-and groove flooring itself laid directly on the joists. I need to do my own excavating in the kitchen--I know there are vinyl tiles (mine), 70s sheet vinyl (there when I moved in) and under that, who knows? From the basement I can see the same t and g boards as in the rest of the downstairs, so I'm hoping it is oak to match the rest of the first floor. My ideal find would be original linoleum in a nice pattern I could use! Whether you could install new flooring as you suggest depends on your joist spacing, the thickness of the flooring, and the weight of your cabinets and appliances. When I moved in, my kitchen had a stove in front of the chimney breast, a refrigerator blocking the only window, and 70s plywoodish cabinets along that wall and forming an ell on the neighboring wall. There was no sag or deflection of the floor. My kitchen is about 12x12, so the span isn't too great, and there is bridging between my joists as in most 1900s houses. I redid my cabinets to cover the wall to the right of the window (found the original doors in the garage and built my own boxes), moved the fridge to the pantry, and replaced the stove with a newer one of the same size in the same spot. Yes, I should have done the floor research at that time, but I just wanted the 70s crap gone. :) That sheet vinyl was under the cabinets, so it was put in before they put the cheap ones in--but it wasn't close to original to the house. If you opt for huge fridges and restaurant size stoves, you may have a problem--but my appliances are standard sizes since I'm just an average person and not a professional chef wannabe. Food tastes just as good done on normal appliances--it is all in the ingredients, not the utensils.

We have red oak floors covered with carpet in our living room in our 50 year old ranch. That room opens up to our dining/kitchen area which is sheet vinyl on a subfloor (placed over old vinyl.) We were wanting to do glued down engineered hardwood over the entire area and the hallway. The guy who measured said we could put a subfloor over the hardwood to bring it up to the level of the vinyl. Sounded easy. Now the installer says we need to remove all the old hardwood and vinyl and do new subfloor over all of it. Who is right? Is a floating installation an easier/better option?

Oops, missed the "engineered hardwood" in the thread title. Why are you using an underlayment with 1" subfloors and engineered hardwood? Isn't the existing 1" built-up? And a "patchwork" of subfloor isn't a problem as long as it's all flush. You should not glue your underlayment down unless for some reason you need to create a 1 3/8" thick subfloor tied to the structure and the coefficients of expansion/contraction are similar for all the materials in the sandwich. Your engineered should have a felt or red rosin slip sheet under it. But to answer the original question, here's APA: