IMO, a sub is the next logical upgrade, even if you think you don't need it. I think this is the missing dimension that has you questioning an amp upgrade in the first place. Even if using it to fill in the very bottom end, it's worth using and learning, especially since your amp has the capabilities to implement one.
ETA: Some room treatment would be a good thing to consider as well, unless you have already done so. Sometimes, these questions/answers can all be for naught if the room is questionable.