Best Upgrade you had?

Speakers, without a doubt. Made the largest, most profound change in my system top to bottom, back and forward--the whole nine. Went from the Revel M20s (which I loved) to the Harmonic Precision Caravelles. Huge difference in $$, but the sound?! My rig is there...got some pretty good electronics/etc. as well.....peace, warren

Building a dedicated room and switching to full-range planers (Apogee Duetta Signatures).

Smaller but still great...adding a tubed pre-amp as a mate with my SS amp.

Dedicated circuts has to be one of my smarter moves also but they were part of my dedicated room build so never had a chance to compare (before and after) results.

Dave

The most cost effective upgrade I ever made was dedicated power lines.

Ok, If we're talking cost effective: I agree with Stage.

For cost effectiveness top flight cans can truly bring out the best of your system without paying exteme dollars for speakers then spending weeks tweaking with room treatments. Also good for isolating what part of the system hums, needs improvement, ect. Followed by dedicated headphone amp with conditioned or battery power supply. IMHO of course.