Best Stereo Upgrade Value

Haven't noticed anyone mentioning power conditioner. EquiTech made every component I have perform much better. Single most effective tweak I've come across.  (not just any power conditioner will do that however.)

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.