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.