Your sub is only as good as the bread it's made on, I don't care how good the meats are at your favorite deli.
Thusly, unless you go to a bakery/deli, where the fresh rolls are baked on the premises…your sub will be sub-standard.
That is all!
You are spot on that the bread is the most variable ingredient in a sub (especially since so many delis use the same meats from a handful of providers).
I disagree that the bread needs to be baked on premises. Although too many delis that get bread delivered serve bread that is already 12 to 24 hours old by the time you order the sandwich, it is certainly possible for a deli to have fresh bread delivered, and turn it over quickly in time for the next deliver. (It is also possible for a deli that bakes its own bread to bake poor bread, or serve when it is going stale.)
Also, what makes a good sub roll is very much a personal preference. Some people prefer the bread to be crustier than others, and some prefer an airier loaf than others. But regardless, the bread should be fresh and flavorful.