It sounds like it is dead. You should get sound from a direct connection if playing music that has bass below the crossover frequency. So if your iPhone connection works with a line level input on the receiver, you can be pretty sure that your subwoofer is at fault. Most likely, it is the amplifier that is dead, and not the actual woofer itself. (This guess is based upon experience and knowing that actual speakers tend to be the most reliable pieces of equipment one has. This is also reflected in many subwoofer warranties, where the amplifier is warranted for a shorter time than the actual woofer. Interestingly, subwoofer amplifiers tend to be less reliable than separate amplifiers, which is either explained by them vibrating from the woofer, or [most likely] because companies go cheap on built-in amplifiers.)
If you are "handy," you can test that by removing the amplifier and disconnecting the woofer from it and then hook up the speaker outputs of your receiver to the woofer and play some music. If you get sound that way, you will know that the woofer itself works. You can then consider whether you want to replace the amplifier, have it fixed, or just buy a totally new subwoofer (which obviously could be done without any further testing).
