It's not the speaker's fault. Your amp has the problem. If you switched the left and right speaker cables I bet the hum would switch to the left speaker. This sounds like DC is getting to your speaker which would indicate a bad amplifier channel. It could definitely cause harm to your speaker (blow the woofer), so I'd have the amp checked out and repaired before using it again. Do you have another amp you could substitute.
Also, recheck your connections and see that you haven't accidentally shorted the speaker connections...
Also, recheck your connections and see that you haven't accidentally shorted the speaker connections...