Re: List Validation: displaying only those values from col A that have B set to value
Is this what you wanted? No array and no helpers. Will only work if the bands are grouped together (all beatles together, all rolling stones together, etc). See attached.
In Data Validation put: =OFFSET(Blad1!$A$1,MATCH(Blad2!$B$1,Blad1!$B:$B,0)-1,,COUNTIF(Blad1!$B:$B,Blad2!$B$1))