Best Upgrade you had?

Early upgrades had a bigger impact; I've been working at diminishing returns in recent years. Probably my best was in 1996, replacing Rectilinear IIIs (bought in 1969) with Thiel 3.6s. The sound improvement was quite noticable. (Heh, heh, heh) Yup.

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.