What's the best way to find a dead cylinder?

Don't discount anything. All this new parts and "professionally rebuilt" stuff means nothing these days. DON'T go thinking " it can't be the problem because it's new".

Seems like you've "eliminated everything " that's been suggested because"it's all new"
Right. How many times have you seen me say, "Just cause it's new, doesn't mean it's good" ? LOL

....and that's SO true, unfortunately even moreso today with the influx of crappy imported parts. This is exactly why I recommend hawking ebay for NOS older American made parts, because the quality is there. That's how I shop for ALL my internal engine parts such as rings, bearings and the like.