I had a x540. It was a GREAT machine. Excellent on hills. And gave a high quality, FAST cut.
I only traded in in because i needed a BX for other projects. The BX isnt half the mower the 540 was.
The x540 has a ground engaging transmission which means it has no transmission issues from pulling weight. I havent pulled 1500lbs with mine however. Ive pulled trailers loaded with firewood and gravel, probably 650-850lbs or so. 1500 is really a substantial amount for a garden tractor, but if the tractor could move it you shouldnt have any issues.