Shower installation with two shower heads each on its own volume valve. The supply into the kohler themostatic valve is 3/4 pex into 3/4 cooper into the valve, exiting the valve is 3/4 copper reduced to 1/2 copper then into each of the 1/2 volume valve and 1/2 copper out to the shower heads, both shower heads seem light on flow both hot and cold (Other fixtures on same feed are fine), Kohler says I should have keep it all 3/4 after the valve but don't most shower heads have to reduce to 1/2 at some point? I was going to replace the valve cartridge to see if it helped but figured I would ask for thoughts before spending the money. Thanks in advance