MuMetal will bring better protection from RF interference, electrical noise, all other electrical and vibration gremlins that exist in such an elaborate device, resulting in more air between instruments, better tonality, refinement. It won't bring night and day differences between an already excellent transformer enclosure SL amps have, but our hobby is a game of subtle, so everything counts in the final sound.
Also if your unit is brand new, you have a long way till it fully burns in.. As I have said at least 1500 hours for the initial stage. So don't judge after 24 hours