Pioneer products with auxiliary inputs vary depending on the model number. EEQ is the performance chip for the equalizer used in many Pioneer players and receivers, and the 50W x 4 refers to the MOSFET50, which is the amplifier chip that provides 50 watts per each of four channels.
Each Pioneer model is different, and those with auxiliary input have locations either on the front of the device or in the back. PioneerElectronics.com has a full list of model numbers with specifications and user manuals available. Pioneer began in 1938 and was founded by Nozomu Matsumoto in Tokyo, Japan, with a primary focus on speaker manufacturing and repair.