There is another way to achieve this with the filters.

Flag all your 1971 songs with a flag called 1971 (nearly one mouse click)
Create a filter 1971 showing flag 1971 as "Prefered". (very easy)
When you are in your Rock playlist, just activate the 1971 filter (still easy).

This is also a way to have multi-genres for a song. Create Flags for each genre. Assigne the different flags to songs and you're done.

DDA