I dont know if its correct.
Damping factor, or output impedance, in tube amps is a combination of the tubes and output transformers. Since the 2 amps use different output transformers its really hard to guess because there are several variables changing at once.
That being said, with most headphones I'd argue that it doesn't matter much either way and more often than many would admit defies common rules of thumb with higher output impedances (lower damping factor) sounding better!
The biggest problem with high output impedance amps (low damping factors) and multi way speakers with passive crossovers is that speaker crossovers are designed around a specific amplifier output impedance - almost always ~0 ohms. When you change the output impedance of the amp you change the crossover frequencies and things get very weird very fast with crossovers that no longer line up and create funky peaks and dips in the overall frequency response. Nobody likes that, and high-output impedance amps get a bad rap.
With single driver speakers and headphones (neither of which typically have crossovers) you have tons of freedom to experiment and highish output impedances often sound much better than ~0ohm output impedances.