My experience with an older Plustek model (7500i), is that it doesn't have very fine control over exposure, compared to the Nikons, and even Epson models. Your experience of not getting any exposure change until 2.0 mirrors mine with that model.
While I didn't end up buying it, I always wondered whether one could take the scanner apart, put a high-quality 0.1 ND (or whatever is needed) over the light source, and then just operate the scanner with Exposure=2.
Other than that, you'll have to rely in the input ICC profile that you create with Argyll to bring those values up to near-white.
Best of luck--
Greg