Best Stereo Upgrade Value

the topic is formulated incorrectly, since in many systems there may be a "black sheep and spoil the whole herd" ...

as a rule - the source is most important (the rest of the modern equipment after a certain price - works satisfactorily and is not so noticeable)

tonearm - cartridge - phono stage have made biggest improvements individually in that order for my system. But if I started out with an excellent source way back, it wouldn't be applicable.

So start with the best you can afford of each component, focusing on the source point through to the end point. A signal can not improve as it goes through the process. You can only get what is picked up by the stylus on the record. Unfortunately it only goes downhill from there, so you should start at the highest place possible.

And to quote a banished former contributor, "everything matters". Except tweaks that don't do anything, or worse yet, degrade the sound.

ODAM capacitors. If you have good equipment, these will turn your system into a great one. Use them as primary, or bypass capacitors and they are a game changer. Linlai E-6SN7's along with ODAM capacitors, if you use 6SN7s are amazing together.

Recently, asking myself this same question caused me to review my favorite source/input device. I stream and listen to CD's far more often than any other including my TT

I upgraded my stand alone DAC with the Denafrips Venus II

I had never before updated any input source with this kind of result. Gobsmacked I am. Great product. Great upgrade.

Mark me down for speakers.  They should also be the most expensive component in your system.  Ultimately, it's the speakers that you are listening to.