Mike, some bourbons from the 70's and earlier have a rum-like taste and sweetness, I'm not sure why. (I hope the old-timers didn't thrown molasses into the fermenting tuns of occasion as a quick way to save money!
). In all likelihood the common factor was congeners resulting from a low distillation proof, but again I'm not sure.
There is a way to reduce the tannin in old whiskey: filter it. Alcohol producers often filter dark rum so well it comes out white. I understand much white rum in the market is a few years old and formerly was amber, i.e., it is rendered white by a fine filtration which takes out the solids Mike V mentioned. There is a white Canadian whisky on the market, White Owl, which is between eight and ten years old. It is rendered white through this process.
So if you could buy a filter like that you can do it yourself, but I don't know what they use for this, perhaps its size or cost precludes home adaptation. If you could do it though, say you took out 20% of the solids: you've reduced the age by 20%, kind of