I think the intent is that they want to prevent people from doubling-up their midbass drivers with additional subwoofers. If you are technically using subwoofers as your midbasses, I don't think that should exclude you. However, the premise of the challenge is a midbass driver showdown, which is kind of specific to midbass drivers by design.
Maybe they could add a category for subwoofers as midbasses?
It is, after all, just for fun as the organizer has repeated many times.
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