Chunky old Den Phal thriving in ... gravel?!?

[mexicowpants]

Me again... My massive old Den Phal (possibly Sonia) has been positively thriving since I acquired it around a year ago, and has recently started to make a bid for freedom - it now has roots trailing down the outside of its pot. Repotting now imminent.
Here's the thing... It's potted in what would seem to be common building site gravel. Yep, gravel. But it seems to like it. A lot. Go figure.
Question is... What the hell should o repot it in? If it's happy in gravel, should I just repot in pure gravel? Or would it do better in something else? Or would changing the medium freak it out?

Another, possibly related, question - I've noticed that a few of the leaves on newer growths are slightly split from the tip along the centre line... What would be the cause of this (my not-so-educated guess would be over- or under-watering)?

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