If I remembered correctly, the 300kOhm resistor is there for the system to detect that you have an Aux connector attached. To an audiophile, this isn't really a good scheme since that resistor will introduce crosstalks between the left and right channels, although at a very low level which can only be detected by an instrument...but still, not a good practice. But the aux input is still much better than an FM modulator. A 300kOhm resistor cutting off low frequencies sounds like BS to me.