I just got a reply from Watts:
Globe valve it is.Thank you for the inquiry. A globe valve would give you more control as far as flow control but any valve would suffice. You need to be able to cut back on the flow to the cold water feed to compensate for the loss of pressure in the hot water feed from going through the heater. You're attempting to equal the hot & cold feeds to the valve. I hope this helps. Thanks again for the inquiry and for your interest in Watts products.
Watts Technical Service Team