Be cautioned about the method being suggested above. If you connect speakers of different impedances to the same speaker output terminal of an amplifier there will be a load and volume imbalance between the Left and Right channels; and worse, it would send your amp into protection mode or damage if the impedance drops lower than what the amp can handle.
We do not know the specifics of the subwoofer the OP is talking about, but know that a subwoofer driver sans a filter can play tones up to 2khz and beyond, it will sound horrid!