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Do you feel like I do? 

Sounds pretty cool, Mark   I've messed w/them for a friend, fixing his vid camera's audio - yes, some can be pretty good little units!

Oh, compression.
In a daw, a tri- or even dual-band comp will get you there fast; otherwise you can play with the idea of a muff-style tone control with a soft-clip pair across the lpf.
Even a baxandall tone control can be "enhanced" this way.
https://patents.google.com/patent/US5509080

I don't know, but I doubt it.

Reducing a cabinet's response to a single frequency response is a big approximation to begin with, since I doubt the frequency response is identical at all power levels, so it should really be some kind of family of curves. Plus, as you say, you can start to get non-linear behaviour too.

My guess would be that when people get into that level of detail for a cab sim, they're working in DSP land.

Since the bands are an octave apart, I was expecting them to be an octave wide, no? Half an octave to the left, half an octave to the right. Instead they are around 0.4 octaves wide according to Jack's calculator.

I doubt that they made the bands too narrow by accident or on purpose, meaning I am not understanding something right. Can someone help me clear this up. Understanding this seems sort of crucial for building a useful EQ.

I suspect this is to do with how Q is defined in bandpass filters. It's the centre frequency, divided by the bandwidth between the lower -3dB point and the upper -3dB point. Imagine what the frequency response looks like - typical bell curve. The part that is regarded as the "passband width" is only the bit between -3dB points at the top of the bell. Since the "skirts" of the bell actually go out quite a bit wider than that (especially for lower-order filters) you might well want to set the Q a bit higher than the raw numbers suggest to stop one band interfering with another quite so much. As usual, there's a trade-off there between having bands that don't overlap too much and having a sound which is unnaturally "peaky".

if the "virtual order" of the filter (in this case defined by the properties of the gyrator) remains the same, then Q and boost/cut are coupled. But that is the same for all EQ designs that I can think of at the moment, no?

Aside from the Rane "Constant Q" design (and probably subsequent copies) mentioned earlier, yes, that's true.

https://www.rane.com/note101.html

I think this is a bit over-played though. Saying that the Q varies with cut/boost makes it sound like the curve gets wider or narrower as you range the cut or boost. I don't think it does, really. The skirt goes out just as wide at high boost as at low boost. What happens is that because of the way Q is defined, the Q works out much lower for a low, rounded hill than a tall mountain. Doesn't mean you don't reach the foothills for both at the same place:

(or see the MT-2 curves below)

There's a picture on that Rane page that explains what a "Constant Q" EQ looks like.

Instead of "squashing" the curve as you turn down the boost, it "shrinks" it.

There's one other issue, which is the coupling between frequency and resonance. The gyrator-based EQ design suffers very badly from this, but not all EQ sections do.

For example, state-variable-filter-based EQs don't have that problem. In the SVF, resonance and frequency can be completely separate. They use 3 op-amps per section rather than the one you need for a gyrator though, so cheaper designs settle for the gyrator.

The Wein-bridge EQ used for mids in the MT-2 also provides a very constant Q as frequency is varied:


Hope these thoughts give you a few more ideas to play with.

T.