Does anyone care that Tuning Fuses have no agency approvals or time/current curves? How do we know if these fuses will protect our equipment?
After having spent 6 years performing safety testing on switching power supplies I would be very concerned about using any protective device, fuse or circuit breaker, that did not have safety agency approvals.
In order to obtain approval by a safety agency, such as UL, CSA, TUV, etc., you must prove that your product will not catch fire or present an electrocution hazard if any component in your equipment fails. This is all that most manufacturers care about. But manufacturers are also often cheap and greedy and often avoid installing fuses that cost them extra money and could possibly cause nuisance tripping. They don't care if every component inside the equipment fails, as long as the equipment does not catch fire or present an electrocution hazard. Quite often the equipment is dependent upon the branch circuit breaker in the building tripping to protect the equipment from catching fire. Most electronic equipment would benefit from the addition of a few extra well placed fuses.
The equipment may have also been designed dependent upon a safety agency approved fuse opening to protect the equipment from catching fire or presenting an electrocution hazard. If you substitute an improper fuse you could be jeopardizing your own safety. As an electronic technician I would never considering installing a non-safety agency approved fuse in customers equipment as I would incur a serious liability if some non-approved fuse did not work properly and the equipment set the house on fire or electrocuted someone. I also prefer to use a well known brand of fuse such as Littlefuse of Bussman. I just don't trust the generic brands.
I won't dismiss the possibility that a fuse can affect the sound. However, the degree to which a fuse could affect the sound is likely dependent upon where the fuse is located. An output fuse or a speaker fuse would likely have the most effect on the sound as the signal travels directly through the fuse with a substantial current. However, the contact pressure of the fuseholder on the fuse is likely to have a more significant effect upon resistance and sound than the fuse itself.
A fuse inside a speaker cabinet is one location where even the elimination of a fuse will not likely cause a safety hazard. It may just be very expensive if a fuse does not blow to protect your speakers. However, an output fuse on an amplifier might be necessary to keep the amplifier safe in the event of a component failure.
I have serious doubts that a fuse on the line side of the amplifier will have any noticeable effect on the sound, unless the fuse is seriously defective. Roger has already outlined how isolated the line side is from the power supply output.
The talk about the directionality of the fuse also seems ridiculous. If there is more resistance in one direction than another that indicates the likelihood of a diode effect. Now consider that your amplifiers are not capable of using the AC power until they rectify it with diodes in series with the transformer secondary winding. The diodic effect of the rectifiers is many orders of magnitude higher than any piece of wire inside a fuse and the diodic effect of the rectifiers would swamp out the diodic effect of the tiny bit of wire used inside a fuse. If there is any measurable diodic effect it is more likely to be caused by the junctions and intermetallic interfaces between the wire inside the fuse and the end caps. It's also very possible that some of these effects are actually thermoelectric effects of dissimilar metals contacting each other.
Fuses also have a DC and an AC voltage rating. When the fuse element opens an arc is formed and current continues to flow, just like an arc welder. When the fuse in installed in an AC line, as soon as the voltage crosses the zero point current ceases to flow and the arc usually stops and no more current can flow and the circuit is protected.
However, it is far more difficult to make a fuse work properly with DC current, because the DC is not switched off to the fuse as it effectively happens 120 times a second for 60HZ AC lines. Instead, the arc that was formed when the fuse was opened can often continue until something physically gets in the way of the arc, such as sand inside a fuse body.
So until Tuning fuses proves to the world that their product meets the various safety agency approvals, and they publish their AC and DC voltage ratings there is no way that I'll use them.