Hi,
I have the WAVTRIGGER and I wish to power an amplifier on the same line. Can I plug the barrel Jack as power in, and go out via the VIN and GND, to my amplifier? (Given that the voltage is obviously correct for both)
Thank you for your answer!
Yes, that will be just fine. 
The VIN and GND pins are connected in parallel with the barrel jack and both have the same voltage on them.
Great! Thank you for your reply!
The Vin pin is directly connected to the tip of the barrel jack. The sleeve is connected to the GND. This means that if you power your Arduino through the barrel connector, then you can use the Vin pin as a direct unregulated battery output.