Yours sounds just like mine, my subwoofer made me feel like I was in it! You're so right, truly understated car that gets looks everywhere you go. I love it.
Yah I need to do some work on my sub and I think my speakers are starting to go bad, perhaps the old owner got it wet at some point of something maybe age but I doubt that.
I think for the price I would take the BMW. You get a proper M car with a reliable motor and 6 cylinders. S2000 is about 5-10k more for similar conditions if not a little more. With that being said however, I have never driven a S2000 and when I do my opinion could change. I want to get a s2000 or at least drive on next year and I will make a video on it for sure.
I have driven both and one an m roadster: The m roadster is a little faster The S2000 handles slightly better when both are stock They are both phenomenal cars.