per John Deere Answer center:
"The general "rule of thumb" is that towing capacity equals total tractor weight + maximum ballast + operator weight. Factory recommended towing capacity is generally determined by the total vehicle weight + operator weight + any additional factory approved ballast such as front weights, suitcase weights or wheel weights."
I recall reading somewhere that maximum ballast is 800 lbs. I know the 3PH spec is 450. So that's about 550 with hitch parts, and then you almost always need front ballast with that much 3PH load, say 250 or so. That would put you at 800 lbs, and is about what I have on it when using my 40 gallon sprayer rig (which it handles with ease).
Based on all this, for an X700 series, I'd guess around 2000 lbs more or less.
Tongue weight should also be considered. On a 3PH, up to about 450 lbs or so is OK. On sleeve hitch, much less.