If a subtropical storm strengthens enough to have 74+ mph winds, then it's getting that type of energy from either a barotropic or baroclinic source, not a messy combination of both. In addition, at that wind speed, it would be pretty much physically impossible for it to not become more fully tropical, since angular momentum would lead to enhanced convection near the center, thus warming the core.
Of course, this example is for a storm that started as subtropical, not a tropical storm or hurricane that has begun to acquire some extratropical characteristics.