Epson V750 with Vuescan won't scan at 6400 dpi

[Noah A]

Title says it all. I just picked up a new Epson V750 to use for test scans and low-res scans to save time and wear and tear on the Howtek.

I'm actually quite happy with the machine so far. I've been using Vuescan and scanning to a 'raw' tiff, then inverting and toning in photoshop. While the scanner may not really have 6400dpi of real resolution, my testing (scanning smaller sections) has shown that the results are noticeably better when I scan at 6400dpi and downsize in photoshop than if I scan at a lower dpi setting.

But I can't get a full 4x5 to scan at 6400. My 6x6 negs scan fine, as do smaller sections of 4x5. There are no error messages and vuescan lets me choose 6400, but the resulting files are actually 3200. And yes, I checked the settings in vuescan, there is no size reduction applied.

Any ideas? Is the scanner capable of 4x5 at 6400?