Great second revival for the topic.
In the years since this started I've tried many different older Whiskeys. That musty taste is more prominent in the Dublin Whiskeys from my experience. Older Paddys didn't have it to the same extent as Jamesons.
I've noticed it in varying degrees with Redbreasts from 60-70's.
It led me to another idea on it. I think there's a combination of things happen. The watering down from cask strength to bottle strength would introduce certain minerals etc. I'm assuming nowadays this is much more efficient.
That musty note could be caused by the interaction of the water and changes in composition and decomposition over time.
It varies from bottle to bottle and over time from the bottles I've tasted. Could certain aldahides/minerals/lactones/esters/congeners in the whiskey be also affected by storage condition and light interaction with these?
I've more of a "database" now and will definitely contact Distillers to get to the bottom of this. More research is needed.