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The C6 has a central obstruction of 37% by diameter while the Dob is about 25%. A larger CO means less contrast plus a less stable view in so-so seeing. Unless the seeing is really good I'd expect the Dob to be crisper than the SCT.
Thermal issues might have been affecting the C6. Have you considered insulating it?
I'd also expect that the brighter images in the Dob might give the impression of being sharper simply because they're easier on the eye.
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