I had all three of the series, didn't have much trouble with any of them.  The 863 was good, but very heavy.  The 963 was so much lighter, only had problems with gearboxes on one row, but only had to check the oil level often (recommended 50 hours, followed that guideline) because one was always low.  Since I filled it with oil frequently, I had no breakdowns, but would imagine I would have had problems had I not maintained it carefully.

I didn't have a drop of trouble with the 1063, and even got careless checking gearbox levels, as I never had to add a drop of oil.  I actually started checking it once a year at the beginning of the season after several years of adding no oil. 

I wouldn't worry about any of these heads, they were excellent, but the 1063 I had was the most trouble free of any of them.  We checked the knives frequently and adjusted them, flipping them end for end before every season and replacing about every other year.  Oil and grease these things, adjust them properly, and they seem to do an excellent job for many years of service.