C-melody Conn out of tune

Hi all C-melody fans!
I have a couple of silver plated Conn straight necks (serial no's within a few thousand in the 137xxx range), and they both have the same problem; The middle C is extremely flat! Using alternate fingering (RH palm key) is a lot better, but still not in tune with the rest. In general the lower stack is sharp and the higher is flat; I can usually compensate for that, but not the C. Necks are original with microtuners, I have a C-mel mouthpiece and use tenor reeds. They are both beautiful instruments but kind of useless right now.
Any ideas?
I've ordered a new C-mel mouthpiece from Aquilasax; anybody have any experience with them? I've also ordered one of his necks and some reeds, so I should probably just wait for his shipment and try them.

Any suggestion is appreciated!