This is worth a read, since I believe you're investing too much worry into this.... Gain staging in the box is a lot more forgiving, since you (usually) don't have to worry about signal loss and added noise.
Long story short, there's a lot more headroom in Pro Tools than what it looks like. And you can track a lot quieter than -6dB, and not suffer from the same issues that tape would have if tracked at a low level.
http://www.rcc.ryerson.ca/media/TECH...8-bitMixer.pdf