Without a doubt, my wife’s wedding ring. She "allowed" me to keep my hand-built 6’-4" Alpine white speakers in "her" Living Room, and a revolving door of boxes, cables, blinking lights and things that made (mostly good) noises.. Just to demostrate how much she loves me, she gave me permission to relocate her framed, signed musical score by Barry Manilow. It was positioned at an an early reflection point in the room and degraded the sound quality.
But, I’d say that the most unexpected, impactful component I ever audtioned in my home in "later life" was a Sutherland phono stage. I had no idea until that day just how much music was being left out of those cheap and/or internal phono stages.