Stackables really aren't the best for 'quiet' anything since they need ~1200RPM fans just to match a single rad.
Stackable config turns the MCR320 more into an "MCR320-GTX"

Gain high RPM performance at the expense of low RPM performance.
EDIT: though I'll admit that fans within a sandwich definitely seem quieter. My two San Aces in my shrouded sandwich at ~2650RPM are quieter than the 1 shrouded one at 2450RPM outside of the sandwich.
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