Thoughts on possible sources of this problem are VERY welcome. Background: New install of "KWC, 1/2" Rough Valve for Thermostatic Trims - K." LINK:

new copper lines to valve. The Tub spout drips (sometimes slow drip, sometimes nothing, and sometimes a small stream of running water). Turning on the sink or kitchen faucet can make the leak worse (or start if not currently leaking). So a change in water pressure affects leak? Plumber thought the valve was bad. Replaced with new valve. Didn't leak at all overnight. Next morning, turned on bathroom sink, then new shower valve started leaking again. He thinks another bad valve. I find this hard to believe and I'm looking for some ideas on what else might be wrong with the install that causes the valve to leak. Thoughts appreciated.