This is a 1x12 speaker simulator based on the Celestion G12H.The frequency response of the G12H can be found here.
It's made up of a gain stage, then 2 low pass opamp filters with resonant peaks to give the slope from about 1.5kHz to 2.5kHz, with peaks at around 2.5 kHz and 4kHz After that, there's a high pass filter to reduce the frequencies from before about 150Hz. The opamps can be any of the types used for audio.
To tell the truth, the Speaker Sim project is more versatile than this and can get very close to the frequency response of this circuit. I've kept this project up because it is simpler and slightly closer to G12H specs than the Speaker Sim.