Really no mystery here at all. As a modestly talented DIY guy I understand that fuses have always been a bottleneck in terms of sound quality. I learned several years ago to build amplifiers and preamplifiers without them. Use a high quality circuit breaker as your on/off switch and avoid fuses in the all important power supply. All that voltage/current going through a tiny, tiny wire connected with cheap fuse holder clamps does degrade the power supply. Seems straight forward in this particular fuse position.
I suppose these aftermarket fuses just limit the power supply less than the common fuse. That makes sense also.
Do one better and bypass those cheap fuse connectors and solder both ends. This makes it harder to swap fuses and direction, but it will certainly help the sound quality.
In digital gear, I just bypass the fuse altogether with great results as fuses are in fact a bottleneck point in terms of sound quality. Many simply could not live with such a mod and I completely understand. The point is the sound quality of these pieces improves with the fuse bypassed as fuses degrade sound.
I do not view fuses as a place where the voodoo claim can possibly reside, at least for the most part. Any fuse manufacturer using technology to improve the passing of voltage, current, and signal through this sound degrading devise is doing good to the component and music that flows out of it. This is no different than upgrading any other component such as a capacitor, resister, rectifier, rca jack, binding post, wire and on and on.
No, this is not at all hard to understand. At least to this simple aphile. Goodness, I upgraded the wire and rca jacks in a passive preamp and improved the sound. I think an upgraded fuse would also be a reasonable place to look for upgrades in performance.