Robert - First off I agree with your view of the calculators as they tend to be cumbersome & far off the street pricing mark, especially the Deere calculator.
Very interested in what you find out is causing your narrow Kubota to ride rough. I'm in the position you were in last year in that I plan to purchase(new or used) a narrow vineyard tractor with cab to coincide with my first 4 acre planting(6500 vines) next Spring. I currently have Deere 5325 which I use for all around land maintenance which will be too wide for my row spacing(7'). I like the Deere but have been seeing quite a few narrow Kubotas in PA and VA. Many of those are the krawler model. A local dealer priced out a new Kubota M8540N cab($44K) and krawler($50K) for me yesterday. My local JD dealer can no longer sell ag tractors, he's relegated to the compact world, so my JD ag dealer is an hour drive. Earlier this year the JD ag dealer gave me a ballpark figure of $45K for the 5083EN cab. I will stop by to see him again to get a firm price. In your narrow tractor search did you consider the McCormick/Landini vineyard series F as they seem to have a longer history of vineyard tractor development? The only hesitation I have with them is the number of viable dealers in my region. Hope you get the Kubota figured out and it becomes a more comfortable tractor for your back. I'll keep checking in to see what you post on this topic......Cheers, Gary