That is one impressive post and an incredible spreadsheet. I think you have really caught the essence of what the Universal EQ is all about. A few points and some answers to your questions.
You have noticed that there are positions for two caps in series on the RLC sections. There are two reasons for this. One is so that you can use one section as an extra RC section as I did in the Helios 69 EQ example. The other is so you can more easily tweak the capacitor values to get hard to buy or unobtainium value capacitors. For example, if you need 18nF but cannot get it you can use a 100nF in series with a 22nF to get it. Or you could use a pair of 100nF in series to get 50nF, or a pair of 22nF in series to get 11nF and so on.
R4 in the inductor calc document is usually VR2. I wrote this document first and before I had got my head around explaining the basic operation of the EQ. I think I did it best in the Conficuration doc. The thing is, R4 does'nt have to be VR2. It is simply convenient to make the basic pot divider as shown in the RC doc but you don't have to do it that way.
The 150R source impedance is what the EQ as a whole is assumed to be driven by. The idea is that the Q of RLC circuits is mostly determined by the EQ components and not what's driving it.
The idea of Rfix is to ensure that the base level attenuation does not change when an RLC section is flipped from boost to cut. So you are correct that the value of Rfix should be the same as that of the pot it is associated with. (this is covered in the Config doc)
Unfortunately the laws of physics dictate that for about 20dB of cut, the top cut shelf frequency must be 10 times the frequency at which cut starts. Shelving EQ is usually described by the shelf frequency not the cut frequency. If you check out most bass/treble controls you will see that the ones specified as so many dB at 10KHz do actually start to cut at 1KHz. You can raise the value of Rshelf but since the cut at the shelf is approximately pot value/Rshelf, you also reduce the cut at the shelf. The thing to remember is that as the pot is varied, the shelf frequency stays pretty much the same and the cut frequency moves. So a 2dB cut that shelves at 10KHz has a cut frequency much higher than a 10dB shelf at 10KHz.
Feel free to ask questions and I will do my best to answer them.
Cheers
Ian
