I am answering differently than most -- alll I need for a great system for all of my own listening (and assuming accees to excellent digital copies without lots of compression -- unlike most pepole), is a set of really good (not elite) headphones like Beyerdynamic 770 or 990 -- good sennheiser or other brands would also be acceptable, and a very good and boring audio card with low noise and diistortion. Possibly an excellent add-on DAC would be nice for the headphones (that extra boost of performance and maybe a bit more convienience when playing material on a portable computer.) Some day, if really needed -- a great pair of monitor speakers might be nice.
The big problem with that system (which is what I have now -- essentially writing software for recording pros) is that getting access to excellent quality digital audio is kind of tricky... So many 'studio' releases are compressed or processed to the hilt -- but I somewhat uncommonly have ways of dealing with the all-too-common over-processed audio.
So, in most cases (other than snake-oil based systems), the biggest impediment to quality is the source (including vinyl because of the troubles with that also) material. (In my case, I judge quality as being 'accuracy' and not necessaily 'over equalized' or being matrixed or timed for a crazy stereo image.)
John