Choose the tip that has the 50 Hz tone closes in loudness to the 500 Hz. The 500 Hz is just an arbitrary reference tone if I'm not mistaken using the Sensaphonics test (takes the variable of insertion depth away). So whichever tip gives you the loudest 50 Hz tone in reference to the 500 Hz is the one you look for. The majority of the time, it'll be just the 50 Hz vs 50 Hz. Although they exist, IEMs that produce perfectly flat accross the 50-500 Hz spectrum are still pretty rare. The majority of the time, the 50 Hz is not as loud as the 500.