patricej

when i do free motion on my 8900 i do these things:

use the screw on the free motion foot to adjust the foot to the lowest possible position.
i crank the top tension up to the highest setting.

i use only organ brand needles.
i start with size 12 and work my way up if necessary.

i did not see any difference at all from switching to the "free motion bobbin case.P
(it did make a difference on my 6600, but none on the 8900.)

i just about never get eyelashes or pin-dots on either the front or back.
if i do, then i know i have to correct the human factor: speed moving quilt sandwich around under the needle.

you will need to experiment to see what works best for you.
the machines might be all the same as they roll off the assembly line, but each quilter is "a unique animal."