I made a music backpack for bike rides using Logitech computer speakers. Power came from 18v cordless drill battery. On the original battery it would play at full, or close to, volume for just over two hours (and they get effing loud). On a lithium ion drill batt was almost four hours. "But that's not 12v and that's what I asked stupid" Yeah, but my point is they seem pretty efficient. I also ran them off the 12v from my BM solar- battery setup and sound just fine. The ac to dc wall adapter says its 14.4v, about the same as a running automobile, and all these type units should be internally regulated to some degree so that's why some varied voltage range can work. I opened it up and soldered directly onto the power-in side to connect to battery. The adapter is still intact to plug into house too. Good luck.
I know enough to know I don't know shit. I just wish more people knew that much.