Modules 550, 550A, 550b, 553, and 525’s all have un-balanced inputs (whereas the 550A-1, 554, and 560’s are electronically balanced input). Read more here. This is usually not a problem when interfaced with something which has a transformer output, like a 312A or 512C mic-preamp. But electronically balanced outputs don’t usually like driving unbalanced inputs, and you may get a 6 db loss and/or some other frequency response, phase response, or headroom problems.
The best way to remedy this is to place Line input transformer before each equalizer. A good transformer will interface with both balanced and unbalanced gear, capable of handling high levels, and maintain excellent common-mode rejection. Good transformers are not inexpensive, but it is the best solution for the best sound.
A good quality 1:1 transformer that behaves well with an 18.5k ohm secondary (the input impedance of 550A/b) should do fine. One option and recommendation would be made by Jensen™ transformers in their PI-2XX if you are looking for an external transformer unit.
Another option is to have us mount Jensen™ transformers inside the R52, which was designed to accommodate this if needed. The extra cost would be $200.00 for this option. Please mention this when ordering. But adding the transformer to the input will only make the R52 for equalizers that are unbalanced and you cannot use your preamps. Also, phantom power will have to be disconnected in case someone does put in a preamp and accidentally engages phantom power, sending 48VDC through the secondary of the transformer.