Realistically, you need both. Monitors for most obvious reasons but headphones when you are doing and checking for intricate nuances, edits and overall effects your drums have on your ears. If your snares (for example) are smackin' to the point where your ears feel like there's a stick being poked in your ears, then you've mixed it correctly. Listen to the Chronic 2001 through headphones and you'll hear and feel exactly what I'm talkin' about. Now your Monitors (and I'm just talking about the drums only here...) are good so you can feel how much air is being pushed out by the woofers and see how they react on your kicks. If you've got some monitors with colored woofers like KRK Rockits, then you can better see what the woofers are doing and hear how your kicks sound and shape your mixes accordingly.
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