Best Stereo Upgrade Value

All other things equal start on cleaning up the power

This topic just goes to show you there are no absolutes in this hobby! The OP asks a question, and he gets almost entirely different answers from most people! This is why we see so many arguments in the forums. I guess everyone's experiences and ears are completely different.

Best upgrade?

Clean your ears. 

Speakers, considering the Dyanaudio Sapphires were substantially bettered in every single way but sheer dynamics when I moved to older used Raidho D2s.

BUT, my DSPeaker Anti-Mode 2.0 room correction unit with DSP took the D2s to a whole new level of superior sound quality in the room. It was just as amazing an improvement, and for the modest money, a far higher value for cost/performance.

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.