Why don't people read the original post? The OP doesn't want a tablet.
Get the biggest e-ink device available which can be rooted / customized. For example, Duokan for Kindle and Prs+ for Sony (though these are for 5 / 6 inch readers only, I belive. Maybe Duokan exists for the DX?). Most out-of-the-box software makes poor use of the hardware.
If there is a large, rootable Android tablet, I believe that would probably be your best bet. Failing that, make sure it's big with decent pdf funtionality.