Buy and assemble what you love and can afford, hell with diminishing returns. Some people will base the change in equipment for the simple reason they love the way a pair of Sonus Faber speaker looks and not necessarily because it sounded better than their previous speaker. Someone may switch from solid state to tubes again for the same reason. Others may change because the new piece of equipment has more features they require. The concept of diminishing returns does not matter if what you changed to brings you more enjoyment and satisfaction.
With all that said, certainly as you spend more for pieces of equipment the next one right above it in the product line may not sound significantly better however if you switch to the higher end line and jump up in models you are going to hear a much more pronounced improvement the majority of time. So once you hit really expensive equipment more than likely to easily hear a big difference means you need to spend a lot more money where if you are in the entry level area the jump in dollars to hear a significant difference is not going to cost you as great a spread in the amount of money required. All this assumes that you have the correct synergy already going with your equipment and your room has been worked on so your equipment is properly placed, particularly the speakers and the corners and walls have been treated.
A better room with treatment is going to make a bigger difference than spending a five hundred to a thousand dollars for a new pair of speakers.