Some writers have made the bold claim that perhaps much of the beloved Sherry influence from pre-90's whisky may very well have come from the use of Paxarette. The now demonized additive was quite common and when you examine what exactly is Paxarette it wouldn't be too much of a stretch to see why the use of PX sherry seasoned casks is now in vogue.
This kind of concentrate use was not unique to whisky, many rum producers would use a kind of concentrate prune "wine" to condition casks, Cognac ages sugar in old casks to use for their dosage along with boiled wood essence.
I'll examine a couple of PX finished whiskys to see if we can find a thread. This Bunnahabhain was distilled in 2003 aged in 2nd fill sherry casks and finished 3 years in first fill PX Butts (which is rare PX hoggies are more common) it was released in 2017 as part of Distell's special rare releases.
Nose: Deep & funky, soy sauce, plum sauce, a kind of meatiness, a slight vegetal side like cooked spinach. With time I get custard (creme caramel), oxidized yeasty wine notes, walnuts and a little putty. With water it brings out more of a nutty side, chestnuts, shoe polish, it becomes more austere also.
Palate: Grapes, apricots, oyster sauce, definitely a bit of sulfur, earthy, salty & sweet. Water smooths it out, makes it creamy, with marzipan but also heightens the saltiness.
Finish: Peppery, more savoury on the tail end, a bit of cooked Brussel sprouts in pork fat. Game meat with juniper and grape reduction. water makes the finish more cardboardy, brings out the tannins in the oak.
Notes: The nose was very reminiscent of the Bunna 12 in my mind the palate is where things take a turn, the PX overtakes everything except the trademark coastal notes, there's nothing subtle about it. I don't know if this is really worth paying a premium over something like the 12 which is well balanced. I guess it's for completists or those who love PX with a touch of funk, not something I would want to own.