Custom. Kelp based potting mix Chicken shit Mushroom compost Dolomite lime Clay kitty litter Perlite Let cook. Its not a super soil but does work well with using small amounts of nutes.
Promix is a good base to add amendments to. But if you can just find regular bales of sphagnum peat moss and the correct amount of lime to add it's real close to promix, then add your own custom blend of amendments.