If all you want is 50Hz boost, use an equaliser circuit.
Just build one circuit for the one frequency.
If you want a circuit to correct for bass deficiency, then I recommend the correction circuit Linkwitz developed. http://www.linkwitzlab.com/
18db of boost requires a lot of extra bass power delivery and bass power handling.
Send in 1W to the normal speaker and you need 63W to handle the bass boost.
If the main signal goes to 50W on a peak signal you now need 3150W capability in the bass department.