Downplayed: Bob's favorite genres just happened to be most prominent in the 1970s, and while his dislike of modern music is never explicitly referenced, it can be inferred through his other interests.
Most people do base their tastes primarily off what was popular or what they experienced when they were younger, at least on some level, and tend to find it harder to be more flexible and incorporating of more modern things once they reach a certain point. In certain cases, this can lead to rejecting new stuff in favour of the old material they're familiar with.
Bob is a time traveller from the past who still finds the future to be a little scary.
The styles and genres that Bob enjoys are no longer being produced much, if at all; he is forced to rely on the past, but would eagerly enjoy modern productions in those favorite styles.
Inverted: Bob prefers modern music to the music that was around in his childhood.
Bob complains about a new album, saying he likes old ones better. But he admits he likes other modern songs.
Alternatively, Bob was born long after the music he likes had its heyday, but still insists that "things were so much better then" despite lacking firsthand experience.
Double Subverted: However, he only likes a few modern songs which ape the stylings of the 1970s.
Bob literally wears a pair of 'nostalgia goggles' which enable him to filter everything around him to what it was like in his youth.
Bob is a Disco Dan who is hopelessly stuck in the past.
In a flashback to the past, we learn that Bob was exactly like this in his youth; he only likes the stuff from the 1970s because it is old now, and hated it when he was younger.
Bob is listening to something and praising it under the belief that it came from the 1970s. As soon as someone informs him it's a modern song, however, he immediately changes his tune, calling it the worst thing he's ever heard.
Zig Zagged: Bob gushes to one guy how much better the music was back in his day...then admits to another that some of it wasn't that great. But when he meets the second guy, he's back to gushing about how oh-so-much better the 1970's were even though this ViewerIsn't A Moron and remembered the previous conversation...when he's called out on this, Bob gets really embarrassed.
Bob never mentions what kind of music he likes.
Bob listens to the music that was popular when he was younger, but is quite willing to listen to modern music as well.
Bob is willing to listen to modern music and evaluate it on an even-handed basis.
Bob doesn't possess rose-tinted glasses when it comes to the past; he likes some of what he grew up listening to, but freely admits that there was a lot of rubbish back then as well, and some of the modern stuff is superior.
Discussed: Two music fans are having a passionate discussion about the relative merits of modern music versus earlier examples, and one of them points out that they need to make sure they aren't just embracing this trope.
Conversed: "There's always one stick-in-the-mud who refuses to believe that anything he wasn't listening to when he was a kid is no good on these shows".
Bob fears the end of his youth and his encroaching old age, and expresses this by lashing out at everything which is modern and which has changed from his youth. All this does is make him bitter and further isolate him from the people around him.
Bob constantly talks about how much things were better in his youth. Turns out, when Alice and Carol investigate Bob's childhood, they find he actually blocked out a great deal of trauma.
After a lengthy process of self-examination, Bob comes to terms with the fact that he's not a young man anymore... but he still hates modern rock music.