One big problem with radiant heat-it is hard to control the temperature. I'd think twice about subjecting the plywood floor to that kind of heat and drying out of the wood. Good Luck, TomC
Tom, it is true that it could be hard to control. Radiat heat of that type is slow to start then will stay warm for awhile. If you just want a warm-up on a cool morning it might drive you crazy. If you need consistant heat in a cooler climate then I think the hot water radiant could be ideal. I'm not worried about the plywood. Remember that hot air movement could dry wood over time, but radiant heat isn't blowing air. It simply raises the temperature. We do it all the time in new and old homes with fantastic results. Even with hard wood floors.
I'm thinking I might put radiant in my walls, then put some small air driven heater cores under some cabinets and such. Hopefully get the best of both worlds.