From the Cirrus article on the subject:
"The consumer electronics industry has standardized to an impedance value of 8 ohms, while the avionics industry has standardized to an impedance value of 600 ohms. While neither is particularly better or worse than the other, they are indeed different. The end result is that power will be lost when a consumer electronics device (with its 8 ohm standard) is plugged into the audio jack on your avionics panel (with its 600 ohm standard). And lost power equates to lower volume."
The impedance matching device works for me. I would imagine some newer audio panels would resolve this mismatch; they know that consumer devices are to be plugged in. Not so on my Garmin.