The secret to a good whisky is good wood, and good wood like good whisky, comes from the smaller craft producers. The quality of the original content of the cask is as important as the quality of the oak used. I always hand pick my casks based on the quality of the wine or spirit it had previously contained. I choose my partner wine producers carefully based on their wine production. I favour, whenever possible, organic wine producers, who’s wine I have tasted, and believe it will compliment the chosen spirit for either a maturation or finish.
The cask will give the spirit it’s colour and essential character, in addition to the natural new spirit flavours.
Historically, in Scotland, whisky was drunk as Uisge Beatha, white spirit, un-aged. Occasionally spirit was stored in wine barrels and mostly French wine barrels. The Scottish court and nobles drunk French wine often referred to as Claret, and when emptied, these casks were used for storing Malt Whisky Spirit. The new make whisky would pick up the rich fruity flavours and tannins from the wine and oak flavin from the wood. Sherry and Port only became fashionable once James V of Scotland became King of England and trade increased between the two countries . At that time, wine was shipped by boat from Spain, Portugal or France and consumed from barrels or jars. The casks were freshly emptied and not treated with sulphur. The freshness of the cask before filling is also an important factor in the quality of the casks used. An old dried up cask will not impair the 
same quality to the spirit as a freshly emptied cask and may leak.
It is a great shame that most producers of sherry see the supply of sherry casks as a business that takes little consideration for the quality of wine within the barrel. This has a knock on effect on the quality of the maturation and finish of a malt.
Hopefully one day people will re-discover drinking sherry in order to help producers make good sherry and supply good quality casks. My favourite is Manzanilla, which when on holiday in Ibiza I tried to order a glass, only to be told they didn’t know what it was and never had heard of Vino de Jeres 😕.