What cutting height are you running it at?
The X models come with a built in stripe kit and it could be whats dragging on the lawn(rubber flap at rear of deck). The original ones were not adjustable but the newer ones are. And you can change the old one over to the adjustable one. I am guessing you are cutting too low for where the stripe kit is set at...hence the dragging and that could also be affecting how the deck cuts.
I would check that out first. If you are dragging it around, raise it up and see if the cut improves.