There is not such a thing as a tailstock spindle mounted in a brass ball bearing. And the bearing mounted in my Premier was a regular radial, (oem numbered but is a ka030cp0). No ceramic balls, just plain steel. Actually the tailstock spindle plus bearing is the same as the d4000. You can remove it and replace it with the other one.
Im now using cross contact bearing with ceramic balls but this is not as important as the wearing in the bearings. Im sure many premiers around are out of specs due to the issue. With balls 0.25 mils smaller the image captured already shows the signs.

Also, not only the dmax is affected by "cross-contamination" between FORI and FOTI. It is also reduced by the poor light sealing inside the optical box. I did try to improve it as much as I could but still is not as as good as the original design, ie d4000.

For the record, I did not hear any of this from Aztek. I gather all the info opening my own scanners.