I recently tried a black QSA fuse (£40) in the mains plug which feeds the LPS to my Bonn 8 Silent Angel router. I really was not expecting anything, having previously added a black fuse to the LPS which feeds my Roon server and heard no difference. The results with this black fuse in the bonn 8 power supply path are startling.
i would just clarify that i have 2 QSA yellow fuses and an orange synergistic in my other power cables so the big jump in SQ may be down to the fact it removed the last :) weak link in the system.
I would encourage anyone with an "audiophile router" and linear power supply to try the QSA black fuse in the mains plug. Really i would encourage you to put black fuses in every power plug but starting with the router is likely to give you a significant uptick.