I have had so little Glenrothes - in fact, my records say only one bottle, also a single cask release from the Scotch Malt Whisky Society ("SMWS"). That bottle was even younger, only 7 years old, but featured very similar casking as this one - initial sherry maturation with a finish in a STR sherry butt, so I am expecting it to be mostly the same.
Glenrothes is a Speyside distillery owned by the Edrington Group (Macallan, Highland Park, etc.), was founded in 1879, and uncommonly features their own cooperage on-site.

