Apogee Duet bass crackling

hi everybody,
think have problem apogee duet (1st gen), i'm newbie when comes sound stuff , want sure it's not misconfiguration or misunderstanding of something. thoughts/ideas appreciated.

problem is: hear bass crackling when listening through sennheiser hd555 headphones more or less bassy songs such e.s. posthumus — nara or positively dark — howl (try it, it's legally free) eq setting in audio player called vox.app:



when plug headphones directly mac , play same songs same eq setting, crackling gone when increase volume.

if turn off eq while using duet, crackling gone well.

think?

 

that's wrong way go eq. if raise gain on of frequency bars, need lower preamp accommodate. example, if raise 64hz 6db, need lower preamp 6db, preamp -6db.

example of 1 of eqs:

 

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