At first I did not understand how this EQ worked; I am always puzzled by circuits that have components connected between the + and - inputs of an op amp - I just can't see what is going on. I have spent some time studying it and now I think I do understand it and it is surprisingly simple. With the pot wiper at the + input end, the input signal is simply attenuated by the pot divider formed by the 27K input resistor and whatever network is connected between the wiper and 0V. With the pot wiper at the - input end, the gain is increased by the parallel combination of the 27K from the - input to ground and the network connected between the wiper and 0V. Because the input resistor is the same value as the one from the - input to ground, the boost and cut are symmetrical. When the pot is at the mid point you get a bit of cut and an equal bit of boost producing an overall flat response.
Once I understood what was going on I realised you could do this with any op amp, including the EZTubeMixer line amp. So I did some simulations and sure enough it works. This means I could do a tube version of the 553 EQ........
Since the EZTubeMixer line amp needs a 2:1 output transformer it needs 6dB gain when everything is flat, and the API circuit has a gain of -8.6dB when I simulated it. I soon realised that you need to change the gain in two parts. First increase the gain in the op amp and second reduce the attenuation in the input. This is necessary to ensure the mid point boost and cut remain the same so the overall response remains flat with the pots centred. So the bottom line is that you need to change both the 27K resistors to trim the gain and they need to be changed by the same amount.
I simulated the trim control across pins 6 and 7 and all this does is reduce the gain still further and only by a couple of dB.
Cheers
Ian