Originally posted by the foolA sad truth is that the string tension will inevitably ruin the guitar, either by bowing the neck beyond what the truss rod can compensate, or causing the body to 'belly' by pulling up on the bridge.
... 12 string Takamine g series (eg335c)- sadly, it is now hardly playable due to the tremendous bowing of the neck. I can only play slide with it :(
Never tune your acoustic 12-string to concert pitch! I recently sold a Daijon 12-string that I bought new in 1977. It stills plays just fine. The reason for this is that it was always tuned in D, two steps down from concert pitch. (D B C F A D) I use a capo on the second fret to restore the pitch to concert E.
My Washburn EA20-12B 12-string electric/acoustic has action comparable to my electric guitars, and shows no signs of getting pulled out of shape.