This is not a startle reflex - this is the beginnings of dominance behavior IMO. Startle reflex would be jumping back and screaming as though scared or running for dear life, not growling and attacking as though pissed someone is trying invade their territory.
Get them both off the couch or any chosen spot, only allow on if they sit nice and ask; start incorporating Nothing in Life is Free type deference exercises, try and find Karen Overall Deference exercises and relaxation protocols, etc. I am sure there is a ton of this type of info somewhere in these forums but I do not have the time to research it for you.
While the telling her off is an okay option - that does kind of reinforce her attitude that boy basenji or cat jumping on = negative things for her - having to move and give up her spot. You could always feed her small treats quickly anytime either boy or cat start to come over but BEFORE she starts to growl so she learns boy or cat = food for me - soon she will be like "come here boy or cat, I'm hungry"
Sorry if it seems choppy/disjointed - in a hurry but wanted to reply.