Keep in mind not all 300B amps/tubes and high efficiency speakers are created equal.
For amps, the output transformer and tube choice, parts choice and bias can make some difference.
For speakers, driver size to match cabinet design is extremely critical for high efficiency speakers regardless of the efficiency and response rating.
And then there is speaker placement, room acoustics issue.
Can't draw conclusion based on few experiment since some has succeed and some failed.
For amps, the output transformer and tube choice, parts choice and bias can make some difference.
For speakers, driver size to match cabinet design is extremely critical for high efficiency speakers regardless of the efficiency and response rating.
And then there is speaker placement, room acoustics issue.
Can't draw conclusion based on few experiment since some has succeed and some failed.