i think it is worth clarifying one important point
diminishing returns does not mean lack of or negative returns
it means as you spend more tranches of the same amount of money (call it $1000 each time), you get a declining degree of positive benefit (in this case, better sound) for each additional $1000 spent - but you still DO GET MORE benefit
said another way, in a world of diminishing returns, so long as it is spent with proper knowledge and insight in the aim to improve, spending more gets you more of what you want
and thus, the controlling idea here is more about how many more $1000 you are willing and able to spend, versus whether those additional thousands will get you further in your pursuit of what you desire...