I have two: When I replaced my pwr cord to a yba diamond on the pwr supply to the audible illusions mod3 pre, and replacing my vandersteens 1b to meadowlark kestrals. Both of these changes were jaw dropping.
Biggest Sonic Improvement
What is the biggest sonic improvement you have ever made to your audio system. Mine was a surprise to me. I replaced my Audioquest PT6 tonearm on my VPI turntable with a VPI JMW Memorial tonearm. The improvement in the sound nearly knocked me off my feet. I never realized a tonearm coud make that much difference in sound. How about some of you guys, what has your most exciting improvement been.