I've observed in a couple of situations that stacking fans on the pull side does seem to work better than one on the push and one on the pull (note - this all at relatively low flow - I'm focused on low noise).

I don't know why this should be true (don't even know if it's true in all situations). After all push/pull should be the same as stacked-pull, only with the radiator fins as flow-straightener between.
If I had to guess it would be that the shrouds we're all using are of inadequate size to be used on the push side (plenum too small)but OK on the pull side.