While I've not heard a 555A in person, word is the pre and power are very nice especially once recapped, and this amp has quite a solid following, well beyond what would be expected of a mid-range model. Not sure I've seen anyone comment specifically on the phono preamp on the 555A, though Sansui didn't really achieve first rate phono stages until around the time the AU-X1 was introduced (circa 1979/80).
All this is not to say the phono preamps on prior models were poor by any means, but it appears most amp and receiver models from roughly 1967-79 were considered as having phono stages that were average to good (at best) and not particularly standouts as with the X1. Of course, Sansui aimed to make a statement with the X1, and they certainly met that objective in every area of it's design, thereby making it a bit unfair to compare it to an older middle of range model such as the 555A.
I'd make the supposition that the outboard phono pre you are using will likely exceed that sound quality of the built in phono stage on the 555A. As to whether is can be improved by modification (beyond simple recapping) is up for debate. I expect some 555A owners will chime in here with their thoughts.
