I reverse-engineered one of these a while back for another company that was interested in re-issuing it, and designed an updated version of it (new power supply, XLR connectors instead of Jones-type connectors, etc.) Unfortunately, the project never happened.
The circuit is basically a replica of the Pultec EQP-1R, with a couple of minor alterations, and a standard Flickinger op-amp circuit as the make-up gain amplifier. The op-amp doesn't contribute significantly to the sound of the EQ, and in reality any make-up gain stage could be used here, like a 990 or even a tube circuit.
I suppose it could be argued that the Flickinger op-amp and Flickinger (Utrad) input and output transformers do contribute to the sound, but I think the vast majority of the character comes from from the passive filter section and the old 50 mH and 100 mH inductors.
The ones I worked on had been modified to be unbalanced input, if I recall correctly, resulting in one less transformer in the signal path. I wonder if yours is wired the same way?
I asked Dan Flickinger about this EQ a while back, and he immediately laughed, saying it wasn't his idea! Apparently the first one was a special request from a customer, and they made another dozen or so just to make some extra money at the time. Dan is very proud of his achievements in terms of the actual
concepts he pioneered in audio, and rightfully so! I think the fact that this EQ is a clone of another design is not something he's particularly proud of. Regardless, it's a great sounding unit and probably will hold its value due to having his name on it, despite the pretty terrible build quality.
Here's a schematic of the design I did, which as stated earlier, is identical to the 351-1 except for the power supply and input and output XLRs.
351-2