Davide, I don't find the 63's as musical as the 57's or the 29xx. Too much top end, and a little digital, to me.
As for protection for the 57's, what about a fast fuse on one side of the speaker terminals? Why not ask Prima Luna about it - they seem to be quite keen on protecting their output transformers, so they should have a very good grasp of what you need.
You can buy fuses from Digikey for a few bucks, and elementary soldering skills will do the rest. A service man could do that for you for not much. Easiest would be to terminate one side of each speaker cable with a fuse.
As for protection for the 57's, what about a fast fuse on one side of the speaker terminals? Why not ask Prima Luna about it - they seem to be quite keen on protecting their output transformers, so they should have a very good grasp of what you need.
You can buy fuses from Digikey for a few bucks, and elementary soldering skills will do the rest. A service man could do that for you for not much. Easiest would be to terminate one side of each speaker cable with a fuse.