Are your hydros completely gone? I just started mine and ran for an hour at 1000 RPM and nothing. Not even steering hydros.
I want to point something out to you that I am not yet absolutely certain about.
This illustration from the WSM shows two hydraulic pumps which have marked #1 and #2. Dont confuse my red numbers with the ones in the illustration by Kubota.
The challenge with the two Kubota WSM's is that they do not show you a completely integrated illustration of the complete hydraulic system. The two WSM are one for the tractor and a second one for the FEL
The one for the tractor does not include the FEL as an example but the WSM for the FEL shows the FEL circuits but not a compete one for the tractor with a FEL.
From what I see the power steering hydraulic pump shares the filters with the main hydraulic pump for the 3 pt and FEL. The PS pump also serves the pto clutch and the hydraulic shuttle.
How many rear remotes do you have and are their controls working freely in the cold? The in cab levers freeze up despite their complicated design.
I am trying to decide if you have one problem or two. One problem could be frozen hydraulic filters as any water could collect there and their blockage would stop both pumps and their respective systems.
If I asked you about a detent do you know what I mean. An example I would give you would be for a woodsplitter. Many have a feature where you don't have to hold the lever to retract the push plate and cylinder. You pull the hydraulic valve lever all the way back and let go. the cylinder retracts and when it reaches the end of its retraction the hydraulic pressure suddenly spikes because the cylinder no longer can move. This spike in pressure kicks the hydraulic valve back to a neutral position.
Remote valves often have detents and detents are not all the same.
A valve staying in a detent position can cause all sorts of problems because there is no pressure for other functions.
Is your bucket self leveling?
Do you have Kubota's easy ride system for your FEL?
I would change the hydraulic filters if you get to that point and then if still having cold weather problems change the fluid. The special fluid is expensive and from your description and hours on your filters since they were changed it does not seem necessary.
You talk about the excellent appearance of your tractor in spite of it being 7 or 8 years old. I wonder if the original owner used a pressure washer on it. A bad practice unless done with great knowledge as water is forced past seals.
Cleaning around the fill plug, pto shaft and 3pt lift arms and linkages could inject water into the main transmission/hydraulic reservoir.
Dave