This is a good thread and here's my input on this, take it for what's its worth.
About 5 months ago I took delivery of my Ultra Platinum modified McCormack DNA-1 to replace a B&K 200.2 which has and still does serve me well.
Needless to say the DNA-1 is a respectable step above the 200.2, I was immediately taken in upon first listening. The DNA comes with standard fuses and I was told the amp responds well to upgraded fuses, I opted to have the amp built with stock fuses so I could add later for comparison.
I let the amp run for a good 2 months or so and decided to try one of the Synergestic Blue fuses, I started with the mains fuse. Now I don't have a dedicated "go to" album or track for critical listening, but I do have a few very familiar albums I use, so somewhat dedicated I suppose.
Peter Gabriel "So" album has some tracks that will let you hear any changes in your system, namely "Mercy Street" and " Don't Give Up". The entire album can be argued that some tracks are mastered for the MTV era which is understandable, but some tracks have many critical passages and nuances that can test your system, as well as strong midbass in some tracks.
I left the lid off the amp after installing the fuse to perform an A&B test of the stock fuse. I did use the "So" album as my first test and listening to Gabriel's and Kate Bush's high vocals in "Don't Give Up", what was immediately apparent was the detail in the upper registry, of their vocals, especially as they taper off each lyric.
I thought, this was placebo so I listened to the track 3 times, then changed the fuse back to the stock. Right away, the upper registry was just high pitched vocals, namely Peters, starting to give way to the limits of my tweeters or the rig in general. I listened a couple more times then changed back to the blue. Again, the vocals just had that last bit of warmth and tone to the highs. Gabriel's vocals retained the grit and slight warm tone all the way to the top and Bush's vocals were delicately soft and liquid throughout. With the stock fuse they were clear, the high notes started to blend together slightly, but missing that level of tonality.
I went back and forth 3 times, results were the same. It really comes down to your system, some may not hear a change, could be what fuse you decide to replace first. I would personally always start with the first fuse inilne with the components input, I chose the amp and it's mains fuse.
This doesn't stop at the vocals, snare drums, ride cymbals, bass guitar, seem to have that last bit of focus and detail and I will contribute that to a cleaner power delivery.
Overall I'm impressed and didn't think a fuse would make an immediate, noticeable difference. However if you think about it, why have all this nice, high-quality copper wire or silver in some places, and feed this with a thin, piano-like wire joined by a cheap metal end cap? That's a lot of current for an amp to pull through a cheap thin wire, why not maintain the flow as efficiently as possible.
I will surely purchase again, I agree the price is a bit more than a normal fuse, but it's a safe bet it's not a normal fuse. Not putting words in anyone's mouth, but just because it's a "fuse" does not mean it falls into the category of cheap, automotive-style fuses that are "just a fuse". IMO, it's another component in the chain that can surely be overlooked.
