Great question, however, I suggest modifying it to biggest sonic improvement per cost. Costliest: Switch from Dual 1241/Shure V15 to Linn LP12/Ittok/Troika turntable. I discovered information (detail) on very familiar recordings I never knew was there! Cheapest: Vibrapods under Linn Tukan speakers. Tighten bass, opened up soundstage and pace. Not bad for $6 ea.
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.