I have been studying the 550, 550a, 550a-1 and 550b for a while now. Not full time but they are all open and I sit and stare when I have a little time to kill. They are all tricky little bastards indeed. The entire secret is in the switches for sure. One of these days I will figure it out but it will not be in the near future. I really need to more fully dissect one but I need time to make sure it can go back together properly. It cannot be sacrificial! ;D Custom Grayhills are probably the only real answer. They will be very steep $$ indeed.
I have heard of a guy out there who has a very good sounding, successful proto version of I think a 550a working. Don't know what his plans are or if he will ever let the world in on it.
For simplicity and since I am sorta of stupid
, I am gathering parts for a nice little inductor based, swinging input eq. I had originally thought of more true 553 but I am starting to lean more towards stepped cut/boost and 2x6 switches varying inductance/caps. An inductor on the high band as well for peak. High and low bands will be peak/shelf. Also with a discrete balanced receiver circuit. That will not be a copy of the input channel receiver but closely related. I will see as I have a lot of other things to get done as well.
Best, Jeff