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

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.

@clearthink I am very well-informed, having an engineering background and work in the broadcast media industry. I know what limits the DC power supply places on sound quality, and fully understand the ramifications of placing a weak link in the chain of power delivery. Your comments betray a lack of understanding of how power supplies function.

You can choose to believe in unquantifiable properties, while I believe in the works of the esteemed scientists who authoritatively laid the groundwork of all modern electrotechnology: Georg Ohm, Heinrich Hertz, James Watt, Joseph Henry, Alessandro Volta, André-Marie Ampère, Michael Faraday, Charles-Augustine de Coulomb, Nikola Tesla, Gustov Kirchoff, Thomas Edison, John Bardeen, Walter Brattain, William Shockley and many others. Believe what you want to, but please refrain from disparaging  others. 

@auxinput I agree, any fuse that comes after the power supply filter capacitor(s) has potential to affect current delivery and hence sound quality. For that reason, I prefer fusing only on the power transformer primary side, where it should have zero impact if not sized too conservatively. Also, I prefer fast acting fuses on everything except the power amps, due to the nature of high and very brief instantaneous current demands. As usual, in keeping with safe operation, as long as the manufacturer recommends a slow-blow fuse, that is the only situation when I would use one. 

@clearthink wrote:

You have the active fantasy life of a small child... from your mountaintop of ignorance, confusion and disorientation.

@clearthink, Would you care to explain your assessment?