A +3/-9 spec would lead me to believe the 50hz is all but un-noticeable as it would be 12db lower at 50, it is there but really making you feel it in comparison to the rest of the music and at 60 it would be 6db(+/-3 from their spec) lower. I would take this as needing a 3-5 db boost in the bass tone control to 'even out' the sound.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70