Like mentioned above, the only Kubota model with DPF that had notorious DPF issues that I could find was the B3350. Those issues seem to be fixed by changes found in Kubota Technical Bulletins. The main reasons for getting the 1511 Z-turn are the air ride seat (for my bad back) and I've got a very good quote on a 1511-72.
Thanks to all for your replies.
Good enough reason. The 1511 has the same engine as the B3350 but expect the 1511 has fewer problems because of the kind of use and doesn't usually get used in winter.