I am not really a "tweaks" kind of guy, with a system that nobody would consider "tricked out". I use a Mach2Music Mac Mini as my digital source through the Krell/B&W/JL chain described in my profile. I believe more in careful system setup and tuning for frequency response and imaging. However, I have experienced a few of the "AHA" moments when unlikely products or procedures changed the perceived sound of my system due to reasons beyond explanation. While I believe that there is a concrete reason for every "change" we perceive, defining and detailing that reason can sometimes become the stuff of legend (at least on some audiophile forums..... ).
With that said, I'd like to relate my experience with a system add-on that works in a manner that is well-documented: vibration control. Since I currently use a second-hand rack with glass shelves that ring like your doorbell, I should have expected that a reduction in vibration would produce better sound. The fact that reducing the vibration in my digital front-end would have a profound effect is what came as a surprise.
I introduce the MAC PLATFORM from Atomic Audio Labs:
I was definitely surprised when I heard the top octaves of my system "open up", with seemingly more frequency extension and "air" without the harshness or tizz one might expect. (There must be a measurable difference here... I could even hear it from the next room.) On recordings with which I am intimately familiar, the change (a very positive one) was unmistakable. This was not a "needs a double-blind test" kind of difference, it was completely apparent in a few seconds of listening.
The build quality of the platform is exemplary. CNC machining produces an exact fit-match to the Mac Mini. The platform sits on three high-quality spikes (under which I placed a small piece of MDF left-over shelving to protect the glass shelf top).
I know that there are other folks here using the Mac Mini as their digital music server, and I would highly recommend that they check out the Atomic Audio Labs Mac Platform. For $299, it made a substantial difference in sound, a difference that I have paid much more for in other system changes over the years. Very nice product!
Now, I truly understand all the interest in high-quality racks and shelving, and the impact it makes on system performance.