We suggest using a TLC2272 for the dual op-amp. The TLC2272 is rated for low-noise and rail-to-rail output, which will offer the highest possible clean boost and dynamic range. The NE5532 is another good choice, but a TL072 can also be used with decent results.
The Mid Shift switch (SW1 on the schematic) selects the center frequency of the midrange dip in the response. The center frequency is moved from around 700Hz (Marshall/Vox) down to 400Hz (Fender) in the HI and LO settings, respectively.
For Fender-style curves, the Mid Shift is placed in the LO setting (closed). To replicate Marshall-style curves, switch the Mid Shift to the HI position (open). Set the Mid control between 12:00 and 5:00 for traditional Marshall settings, with lower settings providing additional flexibility. For a Vox-style response, keep the Mid Shift in the HI setting and set the Mids control fixed at 10:00 and move the Bass between 7:00 and 12:00.
A "flat" equalizer position is set with the Bass and Treble both at minimum, and the Mid control at its midpoint. When Bass and Treble are fully cut, the Mid knob acts as an additional Level control, which is a familiar characteristic of the Fender tonestack.
I've thrown out the previous version as this one is simply better. And definitely verified. There's quite a lot of gain coming from the circuit, so some squeels and noise from the unboxed circuits are expected.