You are not limited to a dual voice coil subwoofer. You can bridge 2 channels which should not cause any more load as long as you do not create any less resistance (less ohms) for each of the channels. If the Sony subwoofer is 1 ohm dvc you should be able to bridge the channels for a 2 ohm svc. Each of the channels will still see 1 ohm. If you don't trust the amp you can get a 4 ohm svc sub and each channel will see 2 ohms instead.