Update - Leak fixed

So the plumber went out yesterday and it turns out the leak wasn't from the Pex tubing at all, it was the valve itself that had a defective somthing-or-other in it. Which means that it was actually Barron Heating who needed to come out and fix it, which they did. There's a shiny new valve now and it all seems to be dry. I left the bucket sitting there for now, just in case, and I'm going to try to make it a habit to check the mechanical room at least once a week in the future so that if any additional problems every come up we'll catch them early. We were just lucky this time!

The defective valve is actually very GOOD news; I was a little worried that if the Pex was having trouble holding up to the heat, where else might it begin to leak? It sure would suck to have a leak in a wall somewhere. Drip... drip... drip... drip... {shudder}