Clean, new common fuses in a pair of monoblock tube amps made a subtle improvement. Synergistic Research Red, Black and Blue (haven't tried the Orange) fuses made/make a HUGE difference. So much so, that the amp designer who detests tweaks, was awestruck and replaced the fuse holders with 5 amp breakers. Now his amps and mine sound very, very similar. The only tweak he approved of was removing the fuse itself. The reason I didn't do the same is that his breakers are NOS and may not be effective in case of fault. I also have an SR blue fuse on my moded Dynaco ST70 to great improvement.
Another friend and I have tried an SR blue fuse on our EAR 864 preamps (not in my main system). Nope, sounds worse than the fuse that came with it. The EAR 890 amp also sounds great (5% difference between it and my custom monoblocks) using the fuse that came with it as well.
My conclusion is that upgraded fuse changes are not always sonically effective. When they are effective, they can be a tremendous improvement in sound despite their "unreasonable" cost.
I use Stillpoints for most of my equipment (Dyanco, monoblock amps, CD Transport, Pre-amp, phono amp, Bryston BIT20 isolation transformer, Blu-ray player). However, they don't make my COS DAC D2V or EAR 890 sound better (actually worse). Why, I don't know for certain. I do know that the DAC was designed with superior vibration isolation characteristics.