Best Stereo Upgrade Value

Upgrade value depends upon your system.  In my modest secondary system (Yamaha AVR, NHT Super Zero Speakers, Pecan Pi Streamer) I recently replaced an old sub woofer with an SVS 3000 Micro sub.  I followed SVS guidelines and changed the subwoofer crossover to 120 Hz while getting the new sub dialed in.  The combination of the new sub and new crossover setting made a quantum improvement in sound for both two channel listening and for movies.

I think that the new sub woofer helped me to maximize the sound quality in this system. 

The singular, most significant item any of you can buy is a USB microphone and measurement program. It terms of sound quality everything else pales in comparison. Those of us who have experience with this die laughing when people say they can do this by ear.

Folks can argue forever but @jmijostyn is correct.

And a simple DSP room (and speaker) correction is one is the most incredible values out there. One should try it and A/B the sound without. It will take what you have to a higher lever. I will never go back.

I really tried my best to get components which were "balanced" with one too much better or worse than the other. This from price, reading a zillion posts and reviews. 

I’m not sure how much of a “value” it was but the one thing I got wrong was my cart (relative to the rest of my system). Going from a Dynavector 20x2 to a VDH Crimson (got it used here so it really was a great value) put the biggest smile on my face. I improved the sound by at least 40% - maybe more.

Just moved in so once I receive my rug and curtains I'm going to measure my room for the first time and treat it as much as I can (that's the short version).

I agree with carlsbad that an average DAC could spoil all the magic. However, intermediate speakers could ruin it.