your most memorable component

what component, other than a speaker, has contributed to the greatest change in the sound of your stereo system ?

what was the nature of the change ?
M.I.T V3.1 cables .......
Thiel CS6 speakers
Sony SCD-1 modded in Frankenstein's lab with 210 amp-hours of battery power, transformer output stage, and the new Audiocom Superclock 4. The far corner of the soundstage is on the sidewalk outside the house.

Mark Levinson 333. I was running my Thiel CS3.6 speakers on a much less powerful SS amp. I learned my lesson...
Quicksilver 300B SET prototype mono amps - I'd never known that kind of seductive presence, and dimension where the music becomes almost tactile.

My first tubed preamp, an Air Tight ATC-1, I never knew at the time that a preamp was so important. Now I'll never go back to ss for a preamp.

My most "recent" memorable component ... Marchand XM126 Crossover. I have been biamping my rig since forever and have heard many times that you need an external crossover to fully realize the potential of biamping. Never got around to check that out. Then one day, I was browsing the 'Gon and saw an XM126 for sale. I thought what the heck, if it doesn't work out, I can always sell it ... It made such a huge improvement on my system I am still recovering from the shock ...
Linn Universal 1.1 player. Gave my CDs incredibly more detail, clarity, and air. A great piece of equipment.
This may show my age, but when I was a senior in high school, I worked all summer and bought my first component, a PE turntable. I didn't have any other components then and spent several months admiring the turntable before I could buy the rest of the system. This was my introduction to "HI-FI". My other memorable component was a Hafler integrated amp. I bought a kit and put the thing together working feverishly every night. To my amazement, when I hooked it up and plugged it in, it didn't blow up and sounded so fantastic compared to my previous receiver. I guess I am getting nostolgic.
Looking into my crystal ball, they will be the George Wright AU-15 2A3 15W push/pull triode amps we just heard. George is on a run through CA, visiting dealers who sell his products, and was kind enough to stop by and let us hear his new 15W EL84 integrated, which is plays far beyond its $1600 retail price would imply. We then heard his AU-15s, which were unquestionably in the top-3 amplifiers my wife and I have ever heard. I'm talking an hour of non-stop goose bumps. Incredible.
Threshold 400a amplifier.
For me,it was discovering Audiogon.
Lamm ML2 monos.
Having my ears cleaned with warm water by a nice nurse.