Depending on the cost, you may find that it's prohibitive. For the sub, the transformer needs to be able to handle the current and that's not going to be cheap. For a small appliance, it's not bad but for something that may draw more current, it's not as practical. If the sub is relatively new, it probably has a switch on the rear for voltage selection- if it does, you only need an adapter for the plug on the power cord or maybe, just a different power cord.
What's the actual model of the subwoofer?