Dead Cylinder Below About 1500 rpm's

Thank you all for interest and advice. While it could be a vacuum leak or a stuck valve - it is likely neither. As I mentioned before, the engine 'shakes' at slower idle. No.6 is not making power. If I speed it up - to over (about) 1500, it smooths out. When I remove the (No.6) lead and 'ground' it, there is healthy and consistent spark. That should confirm the lead, cap and rotor are fine. Cap has one terminal for each lead/plug but rotor affects all.
If a (manifold) vacuum leak, then noticeable at all RPM's, but it is smooooth at fast idle - and above.
Although I haven't replaced the dist. cap, the inside terminals are all fine, and, there's that good, healthy spark - each and every time. So, with fuel, compression, spark and timing, logic has it that there should be ignition and power. There is not much else I can do - - outside the engine. I know of the 'science' that occurs when compressed air/fuel is ignited and how the resulting gasses act upon the compression rings. The one thing that bothers me the most is that, unlike the first time I discovered the broken rings, this time the (cranking) compression is good - too good. Something just doesn't make sense. If the rings are 'intact' - plus everything else I mentioned above - the engine should fire on all six - all of the time.
I will try to find my original points distributor and give that a try - before making any decisions about the 'internals'.