I think its time to ask Kubota how it is going to make things right. IMO opinion this is a considerable defect because you are never going to be able to level out grade or pick up loose materials evenly. Furthermore, you may have other unforeseen issues if you try to run other front attachments such as a snow blower. Did Kubota ever put another loader/subframe on your tractor to determine whether the problem is with the loader or the sub frame? If it were me, I would demand another tractor and FEL because this obviously is a manufacturing defect that they (Kubota) have been unable to address. I'm interested in how Kubota responds to this situation, so keep TBN informed.
You can also remind the service rep that you have posted updates about this issue on a very popular tractor purchaser/user group on the internet and so far Kubota has a favorable image. However, if Kubota decided not to resolve this issue, you would also post that information, and based on that information, many potential Kubota tractor purchasers may decide they would rather have a red, blue, or green tractor. I hope you get this resolved.
Jim