I actually have and use a Pi Audio Magic-BUSS (2x duplexes) in my system that came directly from Dave Elledge (@dBe on the old Mad board and PETT). An acquaintance of mine back in 2009, enquired about it to Dave online in the forums, and Dave sent him one to test out- and it sat for a year on his shelf. Then I found out about it, they both agreed that I take it and try it out.
As instructed by Dave, we talked then on the phone about it, I was to run my refrigerator with it for 2 weeks, and then listen to my system with and without it. It is to be installed in the spot where all runs through it, and it attaches via cord to the single wall outlet. Each duplex is filtered separately, and I was recommended to run sources on one duplex and amplifiers from the other duplex.
So- I ran my fridge with it like suggested. I then took it to Lafayette, IN with me where another friend of mine then resided. He lived right down the street from a power station, and power was pretty noisy/dirty there. I also took my then recently completed MAX 8" 2ways along for the ride. His gear consisted of an HTPC and a Hafler 2200 at the time.
Being I did not focus on proper directivity for these 8" 2ways back then, the MAX had a tendency towards being a bit bright through the mids, and in some rooms they were on the edge of strident, although very dynamic, lively, and capable of higher output levels. They were also a pretty reactive load being what problems I had to solve to get them that far. IE- a less than benign loudspeaker.
We initially set up my MAX speakers and listened for a while, knowing their inherent qualities. Then we swapped in the MagicBUSS to the power line of the Hafler amplifier alone, otherwise same conditions, even used the same source material. After about 5-10 minutes, he looked at me, and I looked at him. The looks on our faces told each other we both thought the same thing, and we were both shocked that it did anything at all, and that it did what it did to boot.
We both felt that the tendency towards stridency of the MAX pair was gone. They were much easier to listen to. It was not subtle at all in that experience. Being forever skeptics, neither of us thought the BUSS would have any effect on the outcome.
When I returned home, where I have a better power grid connection with many fewer problems than in Lafayette, I played with in and out of the system positions for the BUSS. I could tell that the overall noise floor behind the music was lower with it in place than without, and the MAX sounded better here with the BUSS used than without as well. Since I felt it was advantageous, Dave told me I could keep it free of charge. He gladly would have let me return it had I not noticed and liked the difference.
One last thing, that same friend and I have since done an additional capacitance addition to some power strips. Essentially, all it is is an X2 rated capacitor of 0.47uF strung from Hot to Neutral inside a power plug, and plugged into the strip. What this does is filter out noise from the line, and a lot of devices/electronics actually have these installed on the primary side of the transformer for the same purpose. This method works audibly too to a degree, but to less of a degree than the Magic-BUSS did. I still use both options in my system, and he has since picked up the previous model known as the Gizmo (single duplex unit) to use with his system. I also know of 3 more people with them that will not give up their BUSS.
The Pi Audio Group Gizmo, Magic-BUSS, and (current model, and likely 3rd to 4th revision) Uber-BUSS are highly sought after devices. Dave was making them by hand for quite a while, so that is the reason for the simple painted black MDF box in which they typically are packaged. Power cords and cryo blah-blah aside, I do feel the BUSS line of products can improve systems' underlying noise levels to a discernable degree. However, application location may have an affect on how much improvement is possible.
That said, I usually disagree with Danny and his acceptance of snake-oil type products. This is one situation where we agree for the most part.
Does music sound louder when in use? Not in my experience.
Does the noise floor seem lower? Yes.
Are dynamics improved? Not necessarily. It sounds maybe a little "less congested", would be my words.
How much improvement? Depends on your location to the grid, but maybe 5-10% in my location. It's not quite as dramatic as Danny states, but better in Lafayette than here.
Do I know what is in the box? Nope. Dave said it's his life's work, livelihood, and crowning achievement, and convinced me not to open it up.
My 2c....
The old rule of thumb of crossing a tweeter at 2 x the driver's Fs didn't just arrive engraved on stone tablets (or copper plates, as the case may be). 
Lower XO is (may be) OK, but might well require a significant down-rating of the tweeter's power-handling expectation.
it gives more dynamic range to top gun maverick of 2000dB for eye mind exploding and even makes instant coffee
