I don't know why people say lumia 520 didn't get an equalizer because of lack of required hardware.
The SoC used in Lumia 520 is MSM8227 same as Lumia 620 and Lumia 720. This SoC contains Hexagon DSP (QDSP6 v4) and it is responsible for audio processing (Just remember that Nokia can not take this DSP out of SoC ! they can just disable some features of it using firmware). Audio features included in this DSP (QDSP6 v4) are : encoding, decoding, transcoding, noise cancellation, bass boost, virtual surround and other enhancement functions.
So if you see there is no equalizer for 520 then the answer is simple : "It is Nokia policy not to put this capability in the firmware" but hardware wise Lumia 520 is capable of an equalizer just like 620 and 720.