How much subfloor?

I have 2 layers of subfloor but they're somewhat thin, probably 3/8 inch plywood (looks like real wood, not OSB). The joists are 20 inch center-to-center, new 2x8 sistered to old 2x10:

https://sphotos-b.xx.fbcdn.net/hphotos-ash3/558900_4322854502906_180362314_n.jpg

I measured from the threshold (the high point on the inside, notably not to the bottom of the door itself--which is higher) to the bottom of the floor, and got these measurements:

• Front door: 1 7/8 inches to the lowest point I can reach (below the carpet, probably not all the way to the subfloor)
• Back door: Very nearly 2 inches, to the linoleum tile--which I'll remove, but it can't be more than 1/8 inch thick

I'm going to tile the kitchen floor. Thinset, hardibacker, more thinset, then tile. The rest of the first floor gets hardwood, probably 5/8 tongue-and-groove, unfinished Maple if I can find it (I'll polyurethane it with 143A flame-retardant urethane, mainly because I'm told the finish on hardwood is normally AlOx these days and hard to re-finish; best to do it myself).

This raises the question: How much friggin' subfloor do I need?

On top of the presumed 3/4 inch total I have now, I could lay a third and fourth layer at 1/2 inch each and get 1 3/4 inch subfloor. With half-inch cement backer board in the kitchen, 1/4 inch tile, and 1/4 inch of mortar, I'd come right to 2 inches higher than I am now; pull the linoleum out of the equation and I'd still have a 1/4 inch drop from the back door. With the 5/8 inch in the living room, an extra inch would leave 1/4 inch.

The cost and time aren't a problem. The question is is this wrong, harmful, excessive, etc? This is a floor that will hold a 3/4 ton tile floor (maybe more) in a 12 x 12 area with 3cm granite counter tops on top of that near the walls and, of course, one right down the middle on (hopefully) two joists (center island parallel with the joists--actually the entrance to the kitchen gets a new counter top and a 30 inch wide entrance way). There will be a 180 pound piano in the living room, on top of hardwood; I'm sure an extra layer of loose leaf paper is excessive reenforcement in there, but there's no sense in leaving a drop big enough to require a wheelchair ramp for my kitchen to meet ADA guidelines.

And, really, how much subfloor should I lay? Should the floor come right up to the threshold? 1/4 inch below? How much?