The EQ frequency center point of that design is the (referring to the 250EQ DIY schematic) cap across the freq pot pair (refer to the low band section) C27 and the cap going to ground, C28. The tuning of these two caps will determine not only the center frequency, but as you get closer to the optimum center, the band level goes up. As you tune them, you will see the freq shift and the gain go up, and at some point, you will see the gain start to fall down again. That peak is the optimum point with the highest gain and subsequently the end result of the lowest noise. R69 is basically the "summing" resistor into the boost/cut section. When you tune the filters, you can reduce r increase this value to get your correct gain setting.
C27 is the "roll in" frequency and C28 is the "roll-out" frequency. The "bell" switch basically bypasses either the roll in side or the roll out side, creating a shelf. IF the shelf is too broad band, you can also have it switch in a smaller or larger cap into one of the caps.
The basic differenced between the ITI, Sontec and GML is the original ITI, those frequencies weren't optimized as much, then the Sontec used ICs for those amps because Burgess' opinion was that because they were narrow band amps, they didn't need discrete amps. George's opinion was that they did.
The down side of that design is that because you are creating basically an open loop condition for the IC8 and IC7, the noise is quite high. Low noise is best in these positions. When I did the Tonelux EQ, I used this concept (since their are only 5-7 different ways you can do an EQ filter, you have to pick one) but I added 6dB of gain after the boost/cut pot, lowering the EQ noise by 6 dB. It can eat into your headroom a bit, but I never heard a complaint regarding that. Then you can increase the summing resistor R69 and drop the noise the same amount.
Also, if you keep the frequencies of the two low bands at the lowest frequency if not being used and keep the two high bands at the highest frequency when not being used, it tends to minimize the in-band noise.
This is a historical design, and the term "PARAMETRIC" was coined by George in a note to Burgess, left on the door of the lab where they designed it.
referring to this:
https://thedonclassics.com/250eq/schematic/250eq-diy-schematic.pdf
