I'm feeding my Duo with about 6.6VDC, and I find that the PWR and L LEDs are quite dim. Is this normal?
The LEDs are driven from the 3.3V and are rather dim. Check the schematic to see how much current they are taking.
The 6.6v you are feeding the Due goes through a voltage regulator and pared down to 3.3v before getting fed into the chip. The digital pins (I'm assuming you are doing PWM?) are at 3v3, as mentioned by Leon. It is also possible that your resistor (you are using one, right?) may be too large depending on the voltage rating on the LED. Not all leds are created equal.
Axela:
The 6.6v you are feeding the Due goes through a voltage regulator and pared down to 3.3v before getting fed into the chip. The digital pins (I'm assuming you are doing PWM?) are at 3v3, as mentioned by Leon. It is also possible that your resistor (you are using one, right?) may be too large depending on the voltage rating on the LED. Not all leds are created equal.
Thanks for the reply. I was referring to the PWR and L LEDs, which are built on to the board.
Ah, so yes, there is a resistor on those. The documentation for the board says that you should feed between 7v-12v through the barrel jack, since the voltage regulator loses a bit when it steps the voltage down. Does it look brighter when you power the board through the USB? (5v, bypassing the regulator, iirc). If not, then, that's how bright they go. 
-r
One pin is driven from an ARM output, altering the supply voltage won't make any difference to the brightness. Most pins can only supply 3 mA. The power LED is probably driven from the regulated 3.3 V supply, and should remain the same.
The HC pins can source/sink 15/9 ma. The LC pins only provide 6/3 ma. See the "pin out diagram", the first thread in the DUE forum, for which pins are HC. If you want to drive your LED at 20ma, you'll need a transistor or IC