I'm not positive but I don't think it's the bones you are browning so much as the marrow, fat, and any remnants of meat and blood on/in the bones. The bones themselves may not really brown but just have streaks of browned areas where there was blood or meat on them. I agree with Taxy that you can remove the marrow if you don't want to lose it and that you should deglaze the roasting pan. And smaller bones are definitely the way to go because then the water actually reaches all of the marrow. Too large and you may have marrow trapped in the bone that can't add flavor to the stock.
Other than that, I know the best homemade beef stock I've had was made with beef heart that had been browned and then used to make the stock. I haven't tried making it myself this way, just got to eat the results. I've also had soup made with grilled beef and that was pretty amazing too.
One thing my dad taught me about making beef soups is that whenever you cook a steak or hamburger or beef of any kind, if you aren't already going to deglaze the pan for what you are making, deglaze it with just a little water and freeze the results. Keep a container that you add to each time. Eventually you'll have enough to add to a stock or make a gravy or whatever. It's not easy to get a strong beef flavor so this can be a big help.
I find roasted/browned carrots add an amazing flavor to beef broth. I also like browned onion and roasted or browned garlic. Roasted tomatoes or peppers (sweet, green, or chilis, they all work) are really good too. And I like to add something to give it a "green" flavor. Depending on what I'm going for this might be celery, spinach, green beans, or even bok choy. And rutabega is really good in beefs soups. Roasted cauliflower and brocoli are good. Zuchini and yellow crook neck are good too. Oh, sweet potato is awesome in beef soup.
I love bay leaf or rosemary in beef soups. Parsely, oregano, and basil are good too.
A little soy sauce added to the broth can give it a little extra "something". Just use a little so it's a highlight instead of a prominent flavor and remember to reduce/omit the amount of salt you are adding. A little balsamic or cider vinegar can be really good, especially if you are adding sweeter vegetables. Worcestershire or steak sauce can also add a little something extra. I add it to the meat as it's cooking, not to the stock. I think it comes out with a better flavor that way and also gives the meat it's own flavor in the soup.
I also love asian inspired beef soups too so that is another direction you could explore eventually. Add various asian flavorings, spices, and vegetables. Beef soup that tastes like a good shredded beef taco is really good. Add a little lime, lots of corriander, garlic, onion, and maybe peppers, with tortilla strips added just before serving. Or use your favorite carne asada recip and adjust that into a soup.
Sorry, I know this comes a little late for your current soup but maybe it will be useful next time.