I wonder if you are experiencing what I found going from 52 to 252, namely that a balanced and effective pre-amp is not necessarily a component that blows you away out if the box.
I nearly gave back the 252 but the longer I went on with it I realised just how good it was at so many things without any instant or defining characteristics. Every time I swapped back to the 52 I realised that although it was thoroughly likeable the 252 in my system was better and more satisfying in pretty much every way. It took a while to get there though.
I guess a pre-amp facilitates the system in a different way to upgrading the source for example. Every source upgrade I have made was an instant hit.
I have never had a 552 so if this seems like nonsense please ignore! I generally avoid posting about components I have not owned.