Kenny:
Synergistic Research applies 2 million volts to the fuse at a certain frequency and time. They call this process Quantum Tunneling. A descriptive marketing term,in the likes of the Motorola "Quasar" televisions. I have tried several of the aftermarket (but not all ) fuses. I have heard small changes (Isoclean) to large sonic changes( Synergistic Research Black ). SR sells three fuses each with a different sonic change at three price points. I especially like the way my system sounds with SR Black fuses in place. Because the amount and direction of change a fuse can make, I buy from a company that offers a money back policy. I have also experimented with tube rolling in my amps but I know of no tube vendors that have an audition with return policy. That certainly would be welcomed when considering the expensive Elrog, Psvane , KR, or EML tubes.
From an engineering standpoint I do not understand exactly how different brands of tubes, capacitors, and fuses change the sound. But they do.
I have enjoyed taking a considerable amount of time auditioning various fuses and I believe my system sounds better for the time and effort spent.
David Pritchard