Again, I'm no expert but it was my understanding from several distillery tours in Scotland last year, that the best cask for ageing whisky was American Oak that had been used only once to age Bourboun. Sherry wood ageing is usually done closer to bottling for a much shorter period of time. This is because the sherry flavor would dominate the whisky to the exclusion of the other complex flavors. I have also read about Madiera casks and Port casks being used by Glenmorangie for ageing. I don't think it's very typical for an Islay to have very extensive sherry wood ageing (Black Bowmore excluded). This seems to be more of a Speyside tradition.