Sheldon Brown said of JIS "older or lower-quality bikes from Asia". I'd say that your DX-5000 is neither one, so I'd bet ISO. When finding a replacement, stack is important; make sure that your new one has no greater stack than the one it's replacing. Shimano 600EX had a 39.9 mm stack according to Sutherland's, which is on the high side, so shouldn't be hard to replace. Tange Levin (either the plain one, or the Campy look-a-like which is higher) is a good choice. 600EX sets show up often on eBay, but they require those funky wrenches for the flower-shaped top race and locknut.