Best Stereo Upgrade Value

Isolation. Do what you can to isolate each component from the other.  In my case it was getting my speakers off the floor.  I use Townsend speaker bars, but there are plenty of other options. Next turntable then amp. It was incredible. 

positioning your speakers optimally in your room

think of it like properly focusing your telescope

" weakest link " is a great general answer.

In my case, I would say adding PS Audio power regenerator was the single biggest improvement. Before it I was using inexpensive Furman power conditioner.

Next, going from Redgum integrated to Burson separates linked by Wywires Diamond cables.

And third - putting old Purist Audio Dominus power cord on the power amp.

Phono stage, speakers, turntable, in that order.

Weakest link is the place to start.

But if I had to choose, the source is the more important foundation than speakers. A $10K source with a $5K amp will sqeeze every drop of performance out of a $2K speaker and make it sound like a $5K speaker, rather than the other way around, as long as that $2K speaker is not randomly chosen.

Example I have heard:

Furutech passive power conditioner - Melco S100 with LPS - Lumin P1 - CODA CSiB - KEF LS50 Meta - Audioquest Diamond, Yukon, Coffee, Monsoon and Rocket 88 cabling.

Don't love KEF speakers but this setup taught me at least those particular KEFs can sound superb given a clean, powerful signal.