I looked at the website for the speakers (to confirm the rating) and it says RMS power 50-120W. It would probably work though like you said, but I think I wouldn't get the best sound at 30W.

Unfortunately I don't have the manual to the amp anymore. I have no idea where it went, but there is no switch on the amp itself to set power input or anything like that.

From what it sounds like, I don't know why anyone would want to run it at 12V when you can get more power to your speakers at 14.4V.