Sounds like you're on the right track for troubleshooting.
The gas block and tube are not specific to the 300, only the barrel.
What "minimum recommendation" for gas port size did you look at? Plenty of people (and some barrel makers) seem to think they can go by 5.56 specs on the gas system and port size dimensions - that doesn't work for the 300, and you end up with barrels that don't cycle, like yours.
