That voltage is fine, and that voltage drop from battery to amp is fine, nothing to worry about there.
Anything above 12v is NOT low either, especially with the engine off! Thats what it should be. With the engine running they should be around 14v.
How loud is the hissing coming from the amp? Also, what are the settings on the amp itself? It would be easier if you could take a photo of them then I can double check them.
One more silly question, do you only get the noise from the headunit with the engine running, or when its off too? If it happens when its off, you have another problem.
I very much doubt its the amp itself, JBL have quite a good reputation with regards to product quality.
You could always send it back and swap it for another one if you really think its at fault.