Ratio of marrow bones to regular bones to marbled meat?
Marrow bones add a marrow-y flavor. That flavor is very thick and rich, but can taste of innards. Have you ever eaten marrow? (It's delicious, as far as I'm concerned. You can roast and add a touch of salt if you want.) I would say that it sounds like you've got a lot of marrow bones in your stock. That could cause problems, since even a few marrow bones will make the stock fairly greasy. Marrow is almost all fat. 



The ox tails have lots of bones in them. They'll give you the deep flavor and gelatin of bones. (Neck bones give you the same thing, as you can see from the picture below.) The only thing marbled meat would have added is extra fat, which isn't necessary for stock -- you'll end up skimming the fat off anyway. You want both the bones and the meat for stock, as you would for chicken stock. It's true that marrow bones are fairly clean, but they're used for a different flavor. 



Another option for adding lots of flavor is to add what are known as flanken style ribs. This is short ribs that are cut across the bone. They're also used in Korean Short Ribs. They give you some of that marbled meaty flavor, along with the bones from the ribs.