A lot of quality brands give mediocre parts. The fuse might be a piece of Crap wire.
Seriously - if you're paying either side of $10,000 for an amplifier and getting crap parts, then you're being ripped - no two ways about it.
I don't know what fuses I've got in my amp - but I do determine the holistic "overall" by listening, and trust that the manufacturer has used a fuse (whether good or otherwise) which gives the best ROI for that particular product.
That is not to say - of course - that it can't be improved. Of course it can. But more importantly, it is about ROI.
What return on investment can you get by changing fuses? And what return on investment can you get by changing amplifiers?
I think most people will agree that - in most cases - the latter will win every time. No manufacturer is going to sell [say] a $5,000 amplifier when they could command $10,000 for the same amplifier by spending an extra $100 on fuses.
The manufacturer should know what components will sound best in his amplifier. And if he doesn't, then it doesn't really say much for your amplifier design in general. I certainly wouldn't outlay more than $50 on an amplifier if I didn't have total confidence that the partiular piece of equipment was giving me the best ROI possible.
I really don't think big names like Classé, ARC, BMC, Ypsilon, Technical Brain, Lamm, Naim, et. al. are selling amplifiers which could be markedly improved by simple fuse changes!! If these fuses (or other accessories) improved thier products in any meaningful way, by that margin, they would most obviously build them into the design.