You can get an engineered floor that is as good if not better than a solid product. We produce solid unfinished flooring but we also produce unfinished engineered flooring as well. Solid flooring typically has a 1/4" (6mm) of "sandable" or "usable" surface. Basically after you get 1/4" deep you'll hit the nail heads and so you can't sand below that point. Our engineered unfinished product has a 5mm wear layer. However, the engineered product is already pre-sanded to 80g which means once installed the amount of sanding that takes place is minimal. On a solid floor, most contractors will need to do a rough sanding to get the floor even, then one or two more cuts to get it smooth. Even if you have a very talented sander he is going to take 1/32 off in his sanding on the solid floor. That equates to roughly 3/100 which is nearly the difference between the 6mm and 5mm usable surfaces we began with. So in essence, you are getting very close to the same "usable" surface. Not to mention the engineered is more stable, will keep you from having to install a subfloor as it can be glued directly to concrete, eliminates transition issues between hardwood and tile or carpet AND should take less time to install. My two cents. Sam @ Real Wood Floors realwoodfloors.com

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.

Well, if you really want Brazilian Teak flooring and do not want to promote the conversion of the rain forest to pasture or unsustainable agriculture, then source it from someone who does sustainable forestry and not just land conversion harvesting. This is probably easier said than done (especially with products coming from developing nations), as there are many people in the process between the tree and the retail flooring product and some will say/promise things that are hard to verify. Even some of the third party certification systems have found errors in their own verification in developing countries and have had to make changes to ensure integrity. A/C systems, by their very nature, remove huge amount of moisture from the air and lower the relative humidity in a home in the summer to acceptable levels (45%-60%). This is necessary for solid wood flooring that has been dried to 6%-10% moisture content in order to prevent it from expanding excessively in the summer and then contracting again in the fall and winter with drier air. When you say "putting on a humidity control on our A/C system," are you referring to adding a humidifier for the winter on your heat pump? We have a newer heat pump with no humidifier, a finished basement with sump pump, and a moderately tight house and the winter RH is generally the same as in the summer (40%-60%). If you have natural gas forced air heat, it tends to dry the air out more because of the higher temps at the registers and so a humidifier might be needed if your RH gets too low (20%-30%) in the winter time. Again, this also depends on how tight/new the house is. Newer construction tends to be tighter and hold more water inside the home than older draftier houses. The crawl space and related water drainage issues are very site specific.

I've had both tile and hardwood in my kitchens. The tile was very easy to take care of, indestructible, and impervious to spills. However if you dropped a glass, or someone knocked a jelly jar off out of the fridge, smash! into a million pieces. It was hard on my feet and back, and cold in winter -- radiant heat would've fixed that, of course. Hardwood is more comfortable to walk on, warmer even without radiant heat, and more forgiving when you drop something on it. It isn't waterproof if you have a major flood, but a good finish will keep everyday drips from harming it. Sand and grit are the biggest enemies because they become like sandpaper ground in with every step. We have some scratches from the dog skidding around the corners but it's a distressed type of flooring so I don't mind so much. If you are doing radiant heat, what type of wood floor would you put down? Some floors move (expand and shrink) more than others, and radiant heat can make it worse. An engineered floor will move the least. Antique lumber moves some. New wood planks will move the most, QS less so than flat sawn. (These are generalities of course -- dryness of the wood and subfloor, species of wood, and board widths are huge factors as well.)