Aspen has always been my "go to" substrate. It just seems that with this guy, no matter how much I put it, he bulldozes it into piles and has bare floor in places. Funny as heck to watch, but I'm just imagining he'd be happier with something to actually dig in that wouldn't get pushed out of the way as easily.
I had thought about the 50/50 sand soil mix, especially since northern pines are from an area with sandy soil. But I've read a lot about sand related problems so I wasn't sure.