I'm just wondering how they got 30yo sherry casks in there without rejecting some, since it tastes so damn good.
Actually, you needn't wonder. In my experience, sherry casks are in fact more likely to reach older ages than bourbon casks. When I think of the best older whiskies I've tasted (i.e. 30 years or older), they've all been from sherry casks, with the exception of one (namely the 39yo Invergordon).
The reason for this is actually pretty obvious - most of the really good sherry casks are butts, not hogsheads, and we know that larger casks take longer to mature their contents.
So whereas an average ex-bourbon barrel (200 litres) might peak at 18 years or something before starting to show its age, an ex-sherry butt (500 litres) would typically peak many years beyond 18.
(Notwithstanding variations for whether the casks are first-fill, second-fill, etc, etc)
Cheers,
ADmiral