Getty Images Simon Cowell has challenged Maroon 5 frontman Adam Levine to a sing-off! Well, not exactly -- but he has issued an offer to 'The Voice' mentor ... and to the winner of this season's competition. Cowell has suggested an ultimate singing competition, pitting the next winner of 'The Voice' against the future 'X Factor' champ and 'American Idol' winner! Simon presented the idea in a recent tweet, writing, "Maybe the winner of The X Factor Usa this yr should compete with winner of The Voice & Idol in a super final. Just a thought. I'd be up for it." Naturally, the always outspoken Levine had a response for the competitive TV boss on behalf of 'The Voice' -- and he was not too taken with the idea.

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