I have been collecting and drinking single malt for a very long time.
I have heard from many Rabbanim their opinions about the kashrut of the ones matured in wine casks. There are a variety of opinions, and the answer is clear (just as it always is)–ask your own Rav.
(There is no question about the malts that were not preserved in wine casks. This is also why there is no question about the kashrut of bourbon. It is illegal for bourbon to be matured in any cask that has been previously used).
I have also discussed the process of distilling, maturing, and bottling with several distillers as well. Each whisky is matured and finished in one or more casks, some of which may have held wine of some type (including sherry or port). These days, it is very much in style to drink these whiskies that are preserved in wine casks. Therefore, many distilleries say outright on their bottles which kind of casks have held that whisky.
However, not all do. Again: Not all whiskies state the types of casks that they use for maturation and finishing. If they do make such a statement, one may rely upon it. The laws about this in Scotland are extremely strict. However, they are not required to make the statement at all. These days, most distillers and bottler that use casks that have previously contained wine put it on their label. Since these are popular these days, it increases the desirability of the bottle. It is to the bottler’s advantage to label it as such.
However, many brands have been using these wine casks for decades, long before anyone imagined the value of advertising them as such. Their labels have not changed over the years, and make no statements.
If you hold that it does not matter, then no problem. However, if you hold that it does matter, you must be careful. An example: My brother-in-law’s Rav holds that it does matter. When he bought a bottle of single malt for a simcha, he was very makpid that it did not state that it was matured in sherry or port, and he purchased the whisky that was recommended by that Rav. When I got to his house, I noticed that, regardless, it was a bottle that I personally know was matured in a sherry cask. It put me in an uncomfortable social situation, halachically–as the bottle was half gone.
Back to topic: Yes, the sherries, ports and other wines really do change the taste of the whisky.
Now, for those of you who hold that you should not drink something that has been in a cask that previously contained wine but are nevertheless wondering about this taste, I have a solution. There are now a series of malts that are matured in casks that previously contained wine (not sherry). The casks were purchased from the Carmel Wine company in Israel–and the wine inside was kosher. If requested, I can provide the distillery name and suggest places that it can be purchased (It is not that common of a label). Several caveats: 1) The wine in the casks was kosher, and produced under Rabbinical supervision. The whisky–like other single malts–is not produced under Rabbinical supervision. If you only eat or drink product that are–well, this will not help you. 2) This whisky, being a specialty item, is not cheap. 3) Although I had something to do with this process, I am not associated with the company other than being friends with the distiller.