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.
When I replaced my Luxman $500.00 player with CEC 5100Z belt drive.Huge difference.
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.
Mine was going from the Harmonic Tech Pro AC 11 on my Wadia 860x to a Powersnakes King Cobra.
My experience was the same as Ejlif, but in my case I went from HT AC-11 to Powersnakes Cobra. I wonder how much better is the King Cobra.