mdlevine- in my days in yeshiva there was a big debate about scotch whuky-especially blended (as most were)The maskonoh was that whisky is “muttor’ and there was no problem with any additves and-if i remember correctly- there is a teshuva from R’Moshe that there is no problem with the sherry casks, as there is no real sherry in it , no “mamoshes’ and hence no issur of “stam jenom”. don’t rely upon fully uopn what I said because I don’t have the teshuvo handy.
I don’t know why the kashrus organization you mention would single out whisky aged in sherry casks as not permissible. please indicate the source.