Try installing the tubes - especially the leaky ones - and plugs, without the top o-ring, and see if the plug isn't pulling the tube tightly to the head.
The o-ring could be masking a problem there.

Since the plugs are supposed to seat the tubes into the heads, it seems to me the issue is at the bottom of the tubes.
Take a careful look at the sealing area at the bottom: heads & tube face, plug threads.
Either the tubes, or the seating surface, or loose plugs/threads would be the culprit, I think.