Per some of the comments in
this discussion, tell me how you would set up a system with reference quality sound either in a room with a pair of speakers (and subs, if necessary), or via a pair of headphones at the lowest possible cost, but with decent enough build quality to last for more than a few years.
I suspect a lot of answers will be biased towards what people own, because they chose them for this very reason.
With that in mind, lowest possible cost, probably something like the Kali IN-8 V2 for a pair of speakers.
If going passive, with great bass and a very neutral yet detailed sound, Buchardt S400 MKII. If you prefer something a little warmer, go for the MKI. Both of these represent some of the best you can get around the $2k USD mark.
Stepping up and away from low cost, I would look into JTR 3-way speakers with the coaxial compression drivers. Sure, there are other incremental improvements in between these two, but why go to the trouble of changing speakers for incremental improvements? Go big or go home.
And pair subwoofers with everything.
Or, get a really good set of IEMs and save thousands. Or get both so you can have your reference sound on the go and that bigger experience when you get home.