The Octave V16 looks an interesting amp, I have to admit it probably sounds better than my own amp - but there are similarities.
Both use pentodes in triode mode, single ended.
But the KT120 is a powerhouse of a valve! Which explains the authority and good bass performance.
I use the EL33 pentode which sounds very good in triode mode, excellent tonality. I can't use the KT120 etc in my design of amp as it has too low a gain as a triode, so needs a driver stage (hence the 12AU7 in the V16). During development I found that removing the gain stage from my amps improved the sound - but of course it limits the choice of power valve. The EL33 is more powerful than the one I used before, and this is noticeable.
Something I read suggests that the V16 may use an output circuit called parafeed, where a capacitor is used to block DC from the output transformer, allowing this to be smaller and simpler. I've never tried it but I know it can work well, especially in a power amp of low power. Evidently Octave make it work very well.
The V16 is evidently a fine amp, must be well engineered, the specs are good. I'd love to hear one - and I don't say that about too many amps.
As I find the EL33 to be a special valve, my amp might just match it for tonality and vocals, which are generally my priorities.
It's good to find out about how well the Empyrean performs with really good valve amps (I know this already of course), as I seem to remember in the past that it was suggested it wasn't an ideal match for valves!