Lower impedance for AC coupled darlington amplifier?
Your capacitor is making a resistor divider!







simulate this circuit &ndash; Schematic created using CircuitLab

Look at it like this schematic (same thing as your first circuit, darlington transistor as a voltage source, which should be (or at least you want to be) equivalent to your audio signal.

Now, the capacitor is 100uF to DC, but to AC it looks like a resistor of value 40 ohms to a 40 Hz input signal.

40 isn't a lot compared to 1k (your third example), but it is a lot compared to 16 (your first example). Of course, if you short it (second example), then it is zero, and the divider effect goes away.