Wow. Thanks everyone for your suggestions. Bishop8 - it's a fire hazard to have towels too close to a baseboard heater. I believe the distance must be 8" between the top of the heater and the bottom of the towels, which should be fine, except for people and kids who fling the towel over the bar so that it slides down almost to the floor. Billl - yes, that's exactly what I want to do, heat the floor surface not so much the air. You said infrared heater. I think of those when I think of the old-fashioned bathroom ceiling heaters you still see occasionally in motels. Are you saying they make infrared baseboard or wall heaters? Do you know any brands you could point me to? Davidro - as I mentioned in my OP, I am not going to pull up floor tile, because I would never be able to re-do the marble floor tile to match the existing marble tub surround and shower walls. Also, this bathroom is an interior ensuite to the master bedroom - i.e. they share the floor structure, and there are no outside walls in the bathroom. But the bedroom floor feels fine, because it is not marble. It is the marble that is the issue, not the floor structure underneath the marble. Birdgardner - I thought of the attic too, which is actually little more than a crawl space above the bathroom. I checked up there again, and it is quite well-insulated. As I mentioned in my OP, you can feel the marble through slippers, and I should have mentioned they are the thick slippers which are normally warm! I am thinking of chucking the whole idea of a heater (and the cost) and getting a thick rug. Maybe I'll post a new thread on this Forum to ask for suggestions for bathroom rugs. Only problems with a rug is that it won't cover the entire standing area, and I am not a fan of rugs in bathrooms because they are dust/hair/powder/sprays traps. I guess there is no ideal solution, but I must weigh the pros and cons of all the solutions provided here. Thanks so much!

I wish I had known that before now. Unfortunately you often don’t come across these things until there are issues. The GC is suggesting replacing the problem tiles, but doing so may ruin the in floor heating which he’s saying will be our expense. We did the credit card test and there’s about 15-20 that don’t pass.

Found a place online that sells the subfloor panels with grooves to hold the pex line. I'm familiar with electric floor heat, and it looks like the pex lines are a lot farther apart. In a small bathroom, there might only be room for two rows of pex tubing, so I wonder if that will provide sufficient heat. i know baseboards don't provide much of a hot water line, but they have all the fins to absorb and radiate heat. Also not sure if the existing system could be easily converted by essentially reconnecting the water lines from the baseboard to the new pex lines or if other components are necessary. We would definitely have a qualified HVAC person do the conversion, just wondering how complicated it is.

No grout is dirt proof, just stain resistant. You still have to clean it. Buy a high performance grout that doesn't need sealing, like Flexcolor CQ. Use a microfiber mop, soft scrub brush, and a pH neutral cleaner for your existing floor, and then dry it with a microfiber cloth. You may be surprised at how clean it can get. https://www.homedepot.com/p/ZEP-1-Gal-Neutral-Floor-Cleaner-ZUNEUT128/202204696