What I call the Serenity mod is a change to the input biasing method in Naim preamp stages. It's small enough that it does not require PCB replacement, just a few additional components. It is also 100% reversible if that is ever required.
The input biasing method Naim use feeds half the rail voltage to the input to set the DC operating point, this is intended and desirable. Unfortunately it also feeds half the noise, this bad. The only preamp that does not do this is the 552, it's not the only reason the 552 is better than the 252 but it's no small part of it!