PO606 is an ECU memory error. Probably from a low battery condition from attempts to get it started. I would not be concerned with it. To my knowledge, PO606 will not cause any ill problems with the DPF or it's system. It would be good to have it cleared by use of Diagmaster (which is the kubota system for doing this stuff) next time you're in for servicing. It can be cleared from the panel if it's stuck in there but it should go away on it's own after a while...but the ECU will store it forever until it's clearerd.
Surprised it runs at all after shooting ether to it. Seen more than one with lifted heads, broken cranks, bent rods, and now....there is also the DPF to deal with. Rods bend even just a little tiny bit and the engine will still run, and run "good"...but some unburned fuel is getting into the DPF, which then turns into a frequent regeneration cycle, sooner or later it'll go into a level 4 DPF regen, which must be returned to the dealer to them to force a regen. If it goes into level 5, you get to replace the DPF. They will want to know why it was plugged, to prevent from having to do it twice. They find out the rods are bent and find out it was from ether. They send you a bill for about $10,000 to fix it correctly.
These DPF tractors are awesome, but they are nowhere near cheap to fix if warranty can't assist with it.
The old diesels of yesteryear didn't have direct injection and automatic glow plugs, and many also had poor combustion chambers that needed help to get them running. That hasn't been the case since the 1980's with Kubota's and even those old ones would start/run fine down to about 20 degrees F so long as the glow system worked as designed-and was used properly.