My basenjis generally don't like boxers either. Boxers (and labs) are real 'in your face' dogs. Their cue reading skills may not be as sharp as a more primitive dog, like a basenji. So, they are more likely to come right up into a dog's face to say 'hi' without stopping to see if the other dog wants to be friends. Think of humans who are "close talkers"
Personally, I don't think this issue has anything to do with you being "alpha" or not. But you can control the behavior by teaching her "watch" or attention, then not allowing her to carry on by keeping her attention on you when other dogs are present.
And, don't be too quick to think that all basenji growls are aggressive. Some yodels, and grodels sound aggressive, but are just a unique way of showing excitement.