I guess this post fits best in this subforum.
Sure, listeners will get more out of music in general if more appreciation is given to the details and the subtleties outside the main themes, but which composers are especially rewarding the listeners is able to pay attention to all the subtleties? Usually in my case, these composers can be described as "thick" and thematically austere but fantastic in their development of their themes and often change things in their music that is difficult to detect after even multiple listenings, usually multiple listening hint more to the listener just how much depth/ORDER is contained in the music. Brahms is my archetypal composer in this regard.
Some other composers I consider to be in this vein:
Elgar(especially those symphonies that make Brahms look like nursery rhymes, I still don't get them)
Medtner: Very subtle and thick, not as hard as the Elgar symphonies and his first piano concerto actually inspired this post
Taneyev: Austere and clean, cleaner than Brahms in orchestration so a bit easier, but still quite subtle.
Bartok: Dissonant as all heck, but I've found him very rewarding and very tidy
And then for older composers, it maybe works a little differently
Corelli: A baroque composer, but very polished and in a different period sort of way, similar principals may apply.
J.S. Bach: Well, he might be another archetype, but he doesn't always require this kind of listening and I feel it differently for him somehow
W.F. Bach: I have a knack for this guy, but he certainly has such tendencies
I'm curious about how Richard Strauss and Mahler and Bruckner fit into this category. Where are they on this spectrum? And what about Ravel, or late Debussy, or Janacek. What about Gliere?
And what of the early 20th century composers?
And which composers do you think aren't worth all the attention? Just a start to a complex topic.