not sure as I've not had an x but I've seen a few people put on Avon something's which have quiet and ages dive tread pattern and do off roading but I would say gravel with a few rocks is as much as you'd want to go.

as for changing tyres/wheels not sure anyone's went from x to r wheels but there R a few who have bought an r and went to x wheels. search jasxt and look at some of his older posts. means if you get the r you can keep the big wheels for knobbliest and get the x wheels and put some sticky rubber on

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Plough on