On the original lavatube, the outer edge of the top end-cap is raised. I'm not sure about the RBA, but certain tanks will get stopped from screwing in flush with the connector by this outer edge, which means that the connectors aren't completely connecting, and airflow through the vents on the connector won't be able to breathe. If this winds up the case for the Phoenix, then yes, a 510-510 extender would be essential to ensuring that it will function properly on a ver. 1 lavatube.