In this task, you are given a question and a context passage. You have to answer the question based on the given passage.
Q: What was acceptable as music from the 1960s?, Context: Though the music varied widely between regions and artists, the post-punk movement has been characterized by its "conceptual assault" on rock conventions and rejection of aesthetics perceived of as traditionalist, hegemonic or rockist in favor of experimentation with production techniques and non-rock musical styles such as dub, electronic music, disco, noise, jazz, krautrock, world music and the avant-garde. While post-punk musicians often avoided or intentionally obscured conventional influences, previous musical styles did serve as touchstones for the movement, including particular brands of glam, art rock and "[the] dark undercurrent of '60s music".[nb 1] According to Reynolds, artists once again approached the studio as an instrument, using new recording methods and pursuing novel sonic territories. Author Matthew Bannister wrote that post-punk artists rejected the high cultural references of 1960s rock artists like the Beatles and Bob Dylan as well as paradigms that defined "rock as progressive, as art, as 'sterile' studio perfectionism ... by adopting an avant-garde aesthetic".
A:
"[the] dark undercurrent of '60s music"