Aftermarket fuse to tame a bright system?

Fuses have a standards-based rating. Manufacturers of electronics rely on these standards to protect their equipment. You can rely on industrial suppliers like Bussmann or Littelfuse or Eaton to deliver product according to the accepted standards.

One option is solderable fuses. I use them in the power supply for my DIY phono/pre, and there seems to be an audible difference between the solderable fuses and the cartridge fuses. IMO

I’m an ignorant man, and I prefer to use industrial suppliers when I can. YMMV

Yeah, they will help.. I have Synergistic Research fuses in every fuse holder in my system, streamer, DAC, amp etc... They do help with a more analog and smoother sound .. not sure how but they do work and I'll always use them going forward.. I have titanium tweeters which used to be a little fatiguing but the fuses helped, also get a Nobsound Schumann wave generator on Amazon, it's like $50 and also smooths out the system.. and the last tweak that ended my harshness issues was supertweeters, mine are JMR from Japan but they don't make them anymore

Once again, LMAO!

I will never buy an "audiophile" fuse ...   definition of snake oil in my opinion.  

I think your new speaker wires are causing the brightness.