I like Grohe, but my sample of one is not a good sample! The Delta 1700 rough-in valve is often recommended. It will let you use either a thermostatically controlled or pressure-balance valve assembly. I'm partial to the thermostatically controlled valves - set it once, and never worry about it. Typical pressure-balance valves have no volume control, and the temperature will vary depending on the cold and hot water supply. You might have problems if the pressure is not even between the hot and cold inputs - it would look like someone was flushing the toliet, and the valve might respond by shutting down to prevent scalding you. You'd need to read the details on the valve you select to see how it responds to the unique problems often accompanied by flow/pressure issues on a tankless system verses a typical tank.