I've met a handful of bullies that do well at dog parks - they have all been puppies and teenagers. All of our pittie friends (save for one middle aged girlie named Pebbles) have disappeared from the dog park once maturity set in. They were (and still are) great dogs, but with maturity comes clarity of genetics.
Ilsa at the dog park with Jewel... Jewel stopped coming to the dog park at aroung age 2.
Two pitties in the far left, a golden, a lab, and the three dobes - Ginna, Mia, Ilsa.
Our 2005ish Dobe crew at the dog park...
Dobes, especially the females, are pretty reliable dog park dogs - I've known some ornery ones, but for the most part they're good with other dogs. I've never had a problem with Ilsa (aside from other dogs... a westie, a dane, and two border collies attacking HER!)