Depends on your goal. I go for bass impact in my home theater so I suspect mixing models or even brands might be fun there. However in my high end audio system I go for seamlessness of main speakers to subwoofer. I use a REL and tap the speaker outs, as REL suggests. The result is often spectacular and even funny. Turning on the subwoofer makes the system sound as if the main speakers just got better. Virtually everyone says "I don't hear the sub" until I turn it off! In my audio system, were I to add a second sub, I'd want the same make and model.
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