"I'd like to try the SR20 fuses on my REL B1, you mean the fuse 5 Amperes but there are two others on the main map n I have also replaced for best results, or should I replace only one of 5 amperes?"
Hi Pakito, for ultimate performance gains one should replace all 3 fuses but beware that removing the amp/interface from the enclosure to get to the two internal fuses might void your warranty if in the highly unlikely event something should go wrong.
I believe the B1 has two internal 5 amp slo-blo fuses for each power supply rail and a 6.3 amp slo-blo fuse in the IEC inlet but you will have to pull them to be absolutely sure. I would also suggest bumping the amperage rating by a small amount with the new fuses. For example, I would go with 7 amp slo-blo for the IEC inlet and 6 amp slo-blo for the internal fuses.
If you're not comfortable opening the unit to get to the two internal fuses, just changing the IEC fuse gives a nice bump in performance.
Hope this helps.