Just wondering what subwoofer I should get for my new iMac - the Pro Speakers are nice, but I need more bass! I know of the iSub, but just wondering if that's any good, and what else is out there. Also, I assume no matter what sub I get, it will be compatible and be able to physically plug into my iMac? (I use the line-in port for a sub, do I?) Thanks in advance!