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That was the expected result.
Mathematical analysis shows that the S/N is better with darks subtraction.. I posted a link to webpage explaining why it is so couple of week ago.
However, even intuitively you can figure it out:
Any image has signal and noise.. subtracting Dark frame immediately after means adding additional noise to the frame (because subtracting noise has the same result as addition, sign does not matter here).
Stacking process is actually averaging, and because of that fact the noise is reduced. More frames, the better S/N ratio.
After that, subtracting noise stack (again averaged) frame removes permanent sensor faults from final image.
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