@OP - typical 300B amplfiers start to distort above the couple of watts of their rated output. 80dB is pretty loud so, at least on peaks, you will be hearing large amounts of harmonic distortion. As the distortion products are musically consonant, you may not find them objectionable. However, every 300B amp I've ever heard does sound very compressed when driven hard, albeit that this tends to be most noticeable with full range speakers where the most obvious effect is a loss of impact and control in the bass. However, if you like what you're hearing from the combination, then all is well.
Speaker Efficiency
I have been listening to my Decware 300b tube amp for a few months now. At 8 watts, I have them paired with Klipsch Forte IV at 99db efficiency, and this set up has been sounding great. This morning I connected a pair of Sonus Faber Electa Amator speakers, 6ohms at 88db of efficiency. They sound fantastic, with more weight and depth. I understand there is a huge price difference in the two speakers, but aside from that, if I am getting 80db of volume at my chair, 8' away, what is the draw back of using lower efficiency speakers with SET amps, so long as the volume is sufficient? I did notice turn the Pre amp, the tube amp and the Roon volume towards the highest setting, it starts sounding terrible....