When does the law diminishing returns kick in?

As I go through these threads reading responses I will look at the systems from answer writers. Wow, some of you guys don't mess around. As a music lover and audio guy myself (since the late 60s) I can't help but be envious.
Although my system is modest, especially compared to some, I get a lot of enjoyment listening to music on it. It took a while and a lot of trial and error to get what seems right to me. But when looking at the super systems here it makes me wonder what I'm missing. With the exception of deeper bass, am I missing all that much? How much would I have to spend to hear real (worthwhile) improvement?