I've got 550As and 550Bs in my music studio.
They sound great on certain things. Other things, not so much. If used with certain pieces of gear they need line drivers because they load down the signal too much.
I kind of wish they had more frequency options and Qs, but such is the case with most vintage gear. You're not going to use these to cut out offending frequencies, mostly to boost overall high-end or low-end...
I liken them to the GML EQ - they add a bit of a sandy graininess to the high-end when accentuated.
Some things like crash cymbals tend to sound great through them. A voice, however, like a VO or close-miked dialogue, sound horrible I've found.
Good luck in your search.