broadcast centers

Culture spread does stack linearly, allegiance gains/losses do have a cap, however they depend on the culture difference between friendly/enemy. With no enemy culture, one of your broadcasts can max out allegiance gain (or enemy allegiance loss), but as the enemy builds up so must you to keep up the difference between the culture outputs.

So in some cases, multiple broadcast centers in the same gravity well would be a good idea...if I understand that right.

They make culture spread faster, so in reality having more than one is almost always a good idea, especially on the border worlds. Culture spread drops off with each jump, so the more centers you have the farther it spreads in a reasonable amount of time

I don't know, but i think that having mul;tiple boardcast will just increase your culture, or repel harder, just as if you have more trade port or refinary you increase your income, i think centers do the same

btw: how many do you have per planet? most i have was like 3