Don't. It's an artifact of the distance between your sub woofer and the test microphone. How far away was it from the sub woofer when you observed the 50 Hz peak? Listening position, 1 meter away, something else? As you move the microphone, the frequency of the peak & valley change. Trying to EQ it is like playing Whac-a-mole.
Swerd speaks the truth here. In many rooms there's a bass bump and exactly where it occurs and at what frequency are artifacts of the room. You can drive yourself nuts trying to fix something that isn't achievable.
If you hate the sound, or there's something about that bass bump that drives you bonkers that's only a slightly different story. For most applications in most rooms, as Swerd says, the room will have its affect.
Most people forget that a music system is the hardware (amps, players, interconnects, speakers) with software applied (music in either analog or digital format from some source) coupled with the room or listening space. That last part, the coupling with the listening space isn't optional. And if you were to deaden your room to a completely non-reflective and non-reactive set of surfaces, you'd find the sound most distasteful.
WHen I started on this forum, I thought I'd need room remediation and have to spend a lot of money and time on my room since its a theoretically horrible shape for a listening room. With all the help available, I made my room sound freakin' great without much room fixin' at all. YMMV. caveat caveat.
