Q is the ratio of reactive to real energy in a resonator.
Q is the ratio of center-frequency to bandwidth.
In most audio, it is simplest to use John's definition, "spacing between the LF and HF skirts", whenever it gets out of AES subcommittee.
Q < 1 is very mild. Q => 1 is ringy.
In an octave-centers EQ, you can argue for Q of 0.8 or 1.5, but if it isn't close to 1 it will be goofy.
> I don't see how the Q is set. Is that from the tolerance of the actual cap filters and resistors?
You don't "set" something with tolerance.
In a simple L-C-R resonator, you find the L-C resonance frequency, where reactance of L and C are equal, and compare to the resistance. (Some adjustment for high Q, which need not matter here.)
The fake-inductor plan is only slightly trickier, especially if there's numbers on that thing. The "C" is the first cap off the +/- slider, which is marked with a frequency, so we know the reactive impedance of C -and- L from inspection. About 9K. The L-C-R tank resistance is both series and parallel: the to-ground 240K resistor, the 3.9K series resistor, and the boost/cut network. This can be solved easily, but not on one cup of coffee.
BTW: unless AES comes back with an odd definition of Q, the Q of this EQ changes with +/- setting. That's almost universally true, and euphonious. This box's "1-octave" bandwidth is only true at the specified +/-8dB setting. Q is somewhat higher at full boost/cut. However "all" general-purpose octave EQs do it this way, and professional EQs tend to similar extremes. (I've seen $99 consumer Graf EQs with Q>3, to be sure the buyer heard the effect.)
> Urei to make it sound more like the API.
Change the faceplate. That has a major effect on the sound of studio gear.
Can you list the ways the two sound different?
> I think the ICs are complete crap. Hahaha
There are newer, more fashionable, opamps. And '084 can be abused, with high gain, high signal level, and low load impedance. But the Urei did not fall into these traps. And a LOT of GOOD gear, vintage and modern, uses '08X amps similarly without disgrace.
The output amp, where the heavy work happens, is well-buffered and unstrained. Also (unless you run it into less than 500 ohms at over 10V peak) the chip runs class A here.
Measure DCV across R56 and R57, you want at least 25mV across each one. I suspect they won't be equal (OT DCR); that's probably not important.
> the API uses 4558 dual opamps
The 4558 amp is very significantly slower than the '084. 4558 is kin to 741 which gave chip opamps a bad name in audio.
I'm wondering if you LIKE the slow 4558's mush on rapid drum transients. 4156 is in the same class, will replace TL084, but is now rare.