The missing piece

What is a piece of equipment you added to your system and it made such a huge impact you knew you would never get rid of it? cables, components, speakers e.t.c. for me it was a pair of Intuitive Design Gamma Summit speakers.
Probably a tie between the Avid Volvere and the Vandersteen 5s. One expects an audible difference with speakers (voicing, room interaction) so maybe that's a bit of ringer. The Avid was a huge leap forward in analog for me.
Two pieces of equipment for me: 1) Musical Fidelity CD PRE 24 cd player/ preamp and 2) Shure SC 35C phono cartridge.

The CR PRE 24 was the best cd player and preamp I had/ have used. Granted I prefer to buy my equipment new and typically when it is being discontinued and my systems have a definite mid-fi feel to them, still the sound, appearance, and build of the CD PRE 24 has me trying to score a NOS piece, for when the one I have needs to be replaced.

I got back into vinyl about a year or so ago and was just not feeling it and was ready to throw in the towel. I experimented with quite a few cartridges and still nothing, until I tried the Shure SC 35C. Not only is it good for the price, it is just plain good.

Other pieces I got the greatest impact with was a Musical Design t100 amp and the sp1 preamp, in fact I was so impressed with these components I sold my Parasound a21 and p5 so I would have the funds to buy them.
"What is a piece of equipment you added to your system and it made such a huge impact you knew you would never get rid of it?"

Its not about 1 piece that fixes everything. System matching is the most important part of the system. You can have plenty of great components, but if they are not properly matched, you won't be able to hear what they can do.
Oh I hear what they can do. and I agree with you. but one thing I would never part with is the Summits.
For me definitely my Deja Vu Audio El-34 PP tube amp, especially after getting it recapped with NOS Aerovox caps. Another piece I love and can't see ever parting with is my Yamamoto YDA-1 Dac just a superbly musical piece.
Couple years ago I had to give up my listening room which was nicely proportioned for my system. I was forced into a small nearly square room approximately 12x12. Even though I was able to move all my room treatments in, the room was a problem. The biggest problem in the square room was the loss of 3D imaging.  At the time I was using a pair of Thiel 1.6s which in the old room through a very creditable 3D image with depth both behind and in front of the speakers. All gone. The solution was to set the system up in a diagonal configuration. The results were simply amazing. I now have the deepest soundstage I've ever experienced. Instruments as well as vocals are located firmly on a stage in front of me, the speakers are absolutely disappear in the music.  I've tried a few different speakers in this configuration and all but one speaker took advantage of the layout demonstrating exceptional imaging. Not sure how long this room is going to be the system's home. But I'm not looking forward to trying to recreate this sound in a standard room. 
The piece of equipment I recently added to my system is the Silent Line Power Conditioner by Redcore audio. It bested my Synergistic Research Tesla Powercell and my Running Springs Audio Dimitri with the HZ power cord by such a large margin I sold them both and laughed all the way to the bank. The only problem is no one else can ever own a Silent Line Power Conditioner because only 32 were manufactured and built by the Arizona Audio Video Club and no others will ever be made. Such a shame as the parts to build only cost $650 and it blows away all the big boys. We are fortunate to have a club member who is a certified genius and has invented and designed much audio for many manufacturers but doesn't take any credit. I wish you all could try it in your systems. The only thing close to the Silent Line is the Isotek conditioners.
Well, I wouldn't quite characterize it as 'The Missing Piece'; however, I would describe the Studer A820 reel-to-reel deck as adding a wonderful dimension to the listening experience.
Pass 250.8, Synergistic Research cables.
Wonderfull Hyperion HPS-938 speakers. Too bad that Hyperion had no idea how to run business resulting in bankruptcy. Poor customer support, no dealer base (closest dealer to Chicago was in Pennsylvania), no advertising of any kind. Very sad.
First - Boston Audio graphite blocks for speakers. Second - cleaning all the connectors in the system with Mo-Fi two step contact cleaner.
Torus power conditioner . Will stay in every system I put together as long as I live .
I'm still looking for the "holy grail".
Well I've been running various ohm speakers now since 1978 and ohm
Walsh speakers since 1982 or so and don't foresee them going anywhere.
Most other very good stuff I use is probably replaceable one way or
another. The ohms are most unique. Nothing else like them.

Squeezebox touch was a transformational gadget for me as well in terms of
enabling hours on end of musical enjoyment in ways never imagined prior.
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Audio Research Ref 3, let me know I had to upgrade all my components.
It is my belief and experience that most systems do not perform anywhere near potential because they have not been tweaked out properly. Resonance control, power conditioning, flux reduction, room acoustics, CD/DVD treatment, etc. These are the things that bring a system to life. This is the 'X Factor'. Upgrading to a new component can make a difference, but in most cases it will be modest to an already good system. Tweaks take time and experimentation, but typically are the most cost effective things that can be done to find the 'holy grail' as Raymonda calls it.
"t is my belief and experience that most systems do not perform anywhere near potential because they have not been tweaked out properly."

That may be true. BEfore tweaking though comes the choice of components to use together. Tweaking may help but potential cannot be realized unless the parts work together optimally to start. That way you start with good performance and tweak as needed from there to get things sounding the best to you.
I'm in agreement with Mapman somewhat - my madness started when I bought my Naim 5i amp and Gershman Sonogram speakers.

I thought they could sound better :-)

In search of better analogue performance I started tweaking my Rega TT - until the only thing left that was Rega was the lid and power switch.

In search of better Digital performance I switched to a Schiit Bifrost DAC - and proceeded to tweak the heck out of that.

But, the one addition to my system that has paid HUGE dividends in performance was my investment in cables i.e. power, IC's and speaker cables, but it was a very long process

Every cable in my system is responsible for the stunning sound and I'm pretty confident that they have extracted every last ounce of performance from each component.