Two other things:

I gather you do not wrap your hives.  In my experience there are many 
winters where wrapping is not necessary, although, IMO, it always helps. 
The problem is that in northern districts there is always that year where 
wrapping would have made the difference between normal wintering and the 
kind of losses you experienced.  Depending on the region, that winter may 
come once in five or once in twenty.  Sometimes it may come once in three.

The problem is that beekeepers are not very good at understanding that just 
because they got away with something once or twice or because someone else 
says it works for them, does not mean that the idea will work long-term.

I did not ask when you did your last brood chamber manipulations and what 
they were.  One guaranteed method to kill hives or reduce wintering success 
is to rearrange their stores for them too late in the season for them to fix 
your meddling after you are done.  Once the weather gets cool, the wax 
hardens and activity diminishes.

I am quite reluctant to mess with brood chambers after August in short 
seasons and September in a long fall.  I only do that when something is 
clearly wrong, like an undrawn sheet of foundation being found in the middle 
of the hive.

Nobody can arrange a hive for wintering better than the bees do. 

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