Bose 251 outdoor speakers

I'd think it would be real easy to overdrive outdoor speakers...are yours only rattling at high spl's? If so, maybe you're reaching the woofer's limits. More likely, it's probably just a grille vibration or a wire inside the speaker rattling against something. I don't know how hard it would be to open them up for a look see. Worst case scenario - a woofer is shot. Since they are new and under warranty, I would just take them back for another pair.

As for your second question, they are in a pretty crowded field. I'd just recommend carefully comparing what's available and buying what you like.