You already have my comments. ctuna's question is a very good one. I suspect you meant "OEM HiFi amp power".
If I confine myself to your list, better underseats would be my choice. I don't think you will be getting an MS-8 for $400 shipped unless you buy it used. I can't get an MS-8 for $400 shipped, so I don't think that you will.
But if you'd like a deal on my old demo unit, installed in my car for a few weeks, shoot me a message.
musicar northwest portland or musicarnw.com
BMW systems and subwoofers