You can check the driver to see if it's blown. The thing with drivers is that they're either blown or not blown, so if it's ok, then it's not the driver. It could be the attachment to the housing came off or loose and it starts to rattle with low frequencies.

If you're looking to replace your headphones, are you looking for an open or closed design?

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It only rattles at very low frequencies. Does this mean the driver is blown/starting to blow?

Also is there a pair of very bassy headphones anyone can recommend?

If you're trying to get very bassy sounds out of your speaker, then you've probably damaged them. I don't understand Alex's idea that speakers are either blown or they're not. If you have been pushing a speaker too hard it will play music but sound poorly when you continue to push it or it hits the same frequencies that damaged it in the first place. So a driver can be damaged and not "blown". If the product is still under warranty, take it to a repair shop. Most manufacturers will repair/replace a damaged driver one time at their cost. After that they figure you should have learned a lesson if they suspect it was your method of use that might have caused the damage.

You may want to consider Sennheiser HD-590, HD-595 or HD-580. All of them are open models. I've yet to find a closed headphone that sounds better than MDR-7506.

The 555 are ok. In fact, they're actually pretty good, but they don't hold up compared to 595, which are clearer, more refined headphones. See if you can find 590s on the used market. They're the predecessors to the 595's and they're excellent buy for around the same 100-125 you would spend on new 555s.