f = Z/(2*Pi*L) @ 3dB
For LL1623@90mA, the primary inductance is 30H.
F -3dB = 3000 / (2 * 3.142 * 30) = 15Hz
F -3dB = 11000 / 2 * 3.142 * 100) = 17Hz
f = 2Z/(2*Pi*L) @ 1dB
f = 4Z/(2*Pi*L) @ 0.5dB
We need to ensure the phase difference is low and consistent across audio frequency spectrum. We try to have the low frequency phase difference at <20 deg @ 5Hz and <15 deg @ 20Hz. That will give you very solid extended bass. For phase difference above that until 50kHz or more, it should be flat and very low. When both are met, you will get an output transformer frequency response that will satisfy your taste buds.