Hi,
I doubt very much that you will hear any difference at all between the units! Mine had a real mishmash of 2520's in. If there is a difference, you could always convert x/y to m/s eq with you 554's, and convert back to x/y . . . . I have only really noticed a tiny difference with mic pre's at very high gain. I think that a lot of the "sound" is in the transformers, and it is especially the input transformer differences that count in API mic-pre's. There have been a few different input transformers over the years, I think . . .
I have recently built a pair of Audiox's Studer 169-500's, with switchable top and bottom frequencies, no mid band. I have temporarily used two very different 2520's in them, and there is no iron to influence the sound. - 1 original (melville?) and one built using Fabio's board. I was going to swap them out for thew Neumann OA10's that I was going to build. I wont be bothering to do that, since they sound absolutely fantastic, and exactly the same. I guess that in a unity gain situation, that is not too surprising. Even at extremes of boost, they sound the same to me. Using a 5534 instead made it a tad more brittle at the top end.
Also, you could replace the 2520's with 4 of the many types now available. I can personally recommend Peter Purpose for 2520's . . . . I think his are probably the most "authentic", since he reverse engineered an original.
Bon chance!
ANdyP