nad left channel goes out

i have a nad 2400 w/ 1600 monitor series...it will be playing just fine, and all of a sudden left channel goes. if i crank the volume up on pre-amp it comes back (i know you shouldn't do this), my question is, how do i tell if it is amp or pre-amp problem.. i do not want to disasmble entire system, if i can just remove one for repair.
jean