Dead Cylinder Below About 1500 rpm's

It sure sounds like a bad O/F ratio on cyl 6 (probably lean). You are incorrect that a vacuum leak would cause an issue at all rpm's. Exactly the opposite. It would be a fixed added air flow amount (at a given manifold vacuum), so less a percentage as the total air flow (and fuel) increases. Also, at higher power, intake vacuum drops so the leaked-in flow becomes even less. It would be real easy to cap off the vacuum port on the #6 cyl and see the effect, but not sure you are listening to all these great suggestions.