Prometheus was a prototype, and the Defiant was a small gunship. Lots of those might be better than one Galaxy. I don't know much about the Akira beyond it inspiring the NX-01 (or in-universe, being inspired by the NX-01).
We never saw Prometheus in action in any meaningful way. We can assume it has tech that is new, but to take down a Galaxyh class or Sovereign class? We don't have enough info. Considering the latter are hero ships, I doubt it.
Defiant was a powerful ship that could fight well above its weight class, but better than a Galaxy? Not. For all intents and purposes, it was a prototype pocket battleship of sorts, intended for escort duty, that got pressed into service because starfleet couldn't cough up a Nebula or galaxy ship to protect DS9. So they dragged Defiant out of mothball.
If the Defiant was a pocket battleship, then the Galaxy and Sovereign classes were more like the Iowa and Yamato classes, to make an analogy. Unless Sisko is captain and they are using a cloak, a Defiant class couldn't expect to defeat a Galaxy class or Sovereign class under normal circumstances. Absolutely no way in a toe to toe "fair fight."