"I'm starting to feel like we're comparing apples to oranges, but to remain in your context, I would say the opposite. Headroom, the way I understand it, has nothing to do with the strength of your signal but rather how much of a signal - weak or strong - can be accurately captured and reproduced without deficiencies. The more headroom, the hotter your signal can be. Someone correct me if I'm wrong..."
well, if you have a suck pre-amp (like you say the d1600 has) that you have to crank almost all the way up to get a signal, your going to have deficiencies, like distortion that comes from cranking a pot more that 3/4 of the way up. (like a strat thats clean with the volume knob at halfway...then it gets dirty when you turn the volume knob all the way up)
then you take a good pre that gives you the same signal when its turned less than 1/2 way up...your gonna get less distortion.
whether your talking about a P.A. system for a live gig, a guitar amp, or a mic preamp...dosen't really matter.
the amount of reserve gain is your "headroom".
the meek has alot of reserve gain. the d1600 doesn't