Basically, what you are doing then is adding a 410 cab to your combo as an extra speaker. So long as the ohmage on the speaker in the combo combined with the new cab don't drop below the recommended min. ohmage (usually 4 Ohms) then you're fine. If they are both 8 ohm cabs, and your head (combo) is stable to 4 ohms, then go nuts. But if they are both 4 ohm speaker/cabs, and you can't go as low as 2 ohms it'll fry your head.

Hope this helps ya out and isn't TOO confusing...lol

Cheers.

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