I am at almost 300 hours now on the machine. I do sometimes use it at low RPM, For instance when I am using a post driver. I also prefer to keep my hand throttle set down so that I have more control of the tractor with the foot throttle. I try to keep my rpm over 1500 most of the time though.
Ah, foot throttle. So you perhaps rev the engine up and down a lot? Maybe you've noticed on older diesels or anything without a DPF, that a puff of smoke typically comes out when you rev up. This is the soot that clogs the DPF.
Most of us hobby tractor guys on here have smaller hydrostatic CUTs, which can be held at set rpm no matter what speed or what load/job you are doing - less revving up and down. My Kioti has "linked pedal" that lets you control engine rpm with the hydro pedal, sort of like driving an automatic car. But I quit using it because I feared the extra soot generation would be detrimental to my DPF, long term.
No idea what soot loading criteria Kubota has set, but we have a few factors at play here:
1) Cleanliness of combustion - fuel quality, fuel injection precision, injection timing, and airflow. Obviously starting with nice clean combustion is essential and should be a given. But some manufacturers are better than others. I would put Kubota in the premier category here, they know what they are doing.
2) DPF
capacity - A larger DPF unit can absorb more soot before it starts to choke off exhaust flow. But then it will take more fuel, and possibly more time, to burn off in each regen cycle also.
3) DPF soot loading tolerances, and sensor accuracy - need to monitor the DPF properly, and burn it off at just the right time. Too soon and you waste fuel and make people start angry internet forum threads about REGENS EVERY 2 HOURS, lol. Too infrequent, and engine performance will suffer when its clogged, and long term DPF damage is very possible.