Fuse upgrades? -- 6.3 amp 250v 5X20mm fast blow fuses

Before we have a zealot coming in and reducing this thread to a troll fight, I will say that fuses are a way to tune your sound.  There's no right answer and I have actually used different fuses in different situations to "adjust" the sound.
---
Such as, Furutech are probably the most neutral high resolution fuse.  However, too many Furutech in one device can make things too warm/slow.  If you have 4 fuses in the DNA, you probably have a single "Main A/C" fuse.  I would do the 4 fuses all Furutech and then keep the stock fuse for the Main A/C or look at faster alternatives.
----
Hi-Fi Tuning Supreme is a silver/gold mixture and it is a much faster sound with faster attack and more detail.  However, it can easily push a system over the line into the "too fast/bright/harsh" area. 

---
Isoclean or Hi-Fi Tuning Supreme "Cu" (copper).  These are gold-plated copper and can be used to make a system warmer sounding.
----
There are others you can play with, but I haven't tested any of the other new offerings.

Another note.  I haven't tested the Synergistic BLACK/BLUE fuses. The older SR20 fuse is very clean and fast, but it lacks the high frequency response (so it sounds a little like a Class D amp would).
The Synergistic "RED" fuses is about the least offensive fuse.  It will sound okay in any system, but it is slow and not very detailed.  The Isoclean actually gave more detail and resolution, even though it was still a warm sound.

Fuses have no bearing on sound quality unless they are directly between the output stage of the power amps and speakers. Remember, it is the secondary of the amplifier’s power transformer that does all the heavy lifting (typically 5:1 current ratio relative to the primary winding), augmented by the filter capacitors on more demanding load peaks).

The OP’s problem is probably exacerbated by the design spec for his amplifier fuse leaning towards the conservative side, more judiciously protecting the amplifier. I suspect tolerance of the fuse in that case might be a factor also, making the likelihood of blowing a fuse even greater. If it were my gear, I’d take some current draw measurements to get an idea of how close to the threshold the amplifier is running. Consulting with the manufacturer would be prudent, in case they have come across other such cases and deemed that a change in fuse rating is warranted to mitigate nuisance blowing of fuses.

The problem is usually that the fuses (any fuse) cannot be easily made to complete their required job successfully and at the same time handle large transient functions. No one has done it so far, not any ’audiophile’ fuse, either, unless they skirt the edge of proper legal descriptive and actual function of a fuse.

To add, they are highly non-linear, at best....quite large non linear odd order distortions, when under complex dynamic loading, which is what is inside an amplifier.

Tests done with thin wall 99.99% copper tubing as fuse substitutes (found at the given local hobby supply shops), illustrates quite dramatically...just how bad fuses sound. It almost sounds like a double or triple equipment upgrade. It is not subtle. I exaggerate, but not by much. Depends on how the given listener...listens, it depends on what they listen for.

Doing this experiment is not recommended, as one would be incredibly tempted to leave said test devices in place. Unstoppably so. (one would be begging for just one more minute of that incredible sound....!) Unhearing this is pretty well impossible, and will haunt one forever afterward. Do not do this unless you wish to be ruined.

But all of that aside, audiophile fuses sound better than any stock fuse, including Bussman and Littelfuse. Do all audiophile fuses sound alike? Heavens, no! But they’ve come a long way, baby! That’s why audiophile fuses are more popular than ever. You know, with around 30-40,000 in the last few years alone. Hel-loo! Go, ahead, do yourself a favor. Don’t be a cube, rube. Hop on board the fuse train. 🚂 Toot! Toot!

Can’t do this, can’t do that, too complex? Whoa, what’s with all the negativity? Don’t say can’t. Be like the Little Train that could. 🚂 Toot! Toot!