Doesn't matter. You can use either a marble OR (for the OP's benefit) a wood threshold between tile and ANY other flooring material. The marble is thick also, is it meant to be a "bump" in the floor? I just cannot see it being flat to the tile and wood floor Although many people do request that a marble threshold be flush with their floor on atleast the bathroom side, this is actually wrong. Johnmari is right. The whole reason why marble thresholds used to be REQUIRED at all bathroom doors was to provide a small dam at the doorway to guard against minor flooding, from say, kids splashing in a tub, or a minor toilet overflow. Mowers-- you're doing fine. You were right to install that plywood. What you had before wasn't NEARLY enough, and what you've put down now brings it up to where it should be. One thing to think about-- instead of using 1/4" CBU, what about using Ditra instead? It's only 1/8" thick, as opposed to the 1/4" of the CBU, and it'll provide alot more protection against movement to your floor. As for the heating mats, I don't see a problem with that, either (OVER the Ditra, not under). About the only thing you'll have to do is undercut your bathroom door, and maybe use a flange extension on your toilet flange, if it's already set.

Viola Park is a line a cabinets that is owned by Henrybuilt. It is made in the same factory using same processes but cheaper: they use fiberboard (not sure the exact name) instead of plywood and MDF slab doors. These are great quality cabinets also. Viola Park uses slab painted cabinets extremely well for stylish look. I think you should go for a fashionable color; grey with punch of accent. Viola park does these amazingly wonderful colorful accent pieces! I have been looking at many many old houses recently to buy. Most older homes from 1920 to 1950s that still have orignal cabinets intact (which are many in Seattle!) have slab painted cabinet doors that are mounted over framed site built boxes. The almost all have vent holes in the sink cabinets. (why? they thought garbage needed airing!) I have yet to see an older home with original shaker style cabinets in the kitchen. They are however, in the original built ins... Perhaps, they thought they were saving money by not putting in the nice stuff in the kitchen. who knows. I think you can't go wrong either way. I happened to really like the simplicity of slab doors. My 1960s rental house has slab cabinets with framelss boxes, euro exposed hinges of that era and the cabinets are still being used. The are still straight and extremely functional and stylish looking and they work well in the house. They are currently painted white. I may paint them soft grey next time to make them look "modernized". My dentist's office has turquoise/teal colored painted slab cabinets from 1960s with flat tongue and groove cedar paneling accent walls. The design is an amazing example of MCM! The dentist who has really good eye kept the design when he expanded. I know that you do not want MCM but I just wanted to let you know that the slab doors extremely versatile. Here is a link that might be useful: Viola Park

I repaced my front hallway flooring with Italian porcelain wood look tile. The hallway transitions to my kitchen, which had Pergo wood look laminate flooring ( which Pergo discontinued ). My carpenter fabricated a simple wood transition piece between the 2 floors.

I have only one door where flooring meets tile, and the transition strip T molding is directly under the door so when the door is closed the transition piece is not visible. Hope this helps!