According to Ibanez, the Phat II EQ circuit provides an "active bass boost" which "offers additional low-end power." The Phat II EQ came as a standard feature on my black GSR205 when I purchased it in 2006. Onstage, the circuit lived up to its promise, providing a discernible bass boost controlled by a dedicated B20K pot. The Phat II also seemed to boost the highs and reduce the mids.
The only problem was that along with the boost came an intolerable amount of noise, thanks to the single coil pickups. This problem, plus the lack of a bypass switch, prompted me to remove the Phat II EQ and rewire the GSR205 along the lines of a Fender Jazz Bass (see Mod #1). Hope you enjoy the pics :).
A. Before Desoldering the B20K Pot
B. After Desoldering the B20K Pot
Front view of circuit board: visible markings
Front view of actual circuit board
Closer view of 1st segment
Closer view of 2nd segment
Closer view of third segment