the 1300 valve is a pretty good one for the money
its not their best but seems to work pretty well...
if you can cut an access panel behind that faucet in a closet
or drywall in a hallway, you are probably going to find the pipe going
up to the shower head has not been soldered too good at the brass connection to the faucet..... thats a good guess anyway...
take the face plate off the faucet and look inside through the hole
if it is large enough.... if it is not , you can expand that hole in the tile
out to about 1/2 inch under the size of the face plate and possibly check it
out through there... and your contractor might be able to re-solder that
fitting through the front (if he knows what he is doing)