Phil I tried many Audion amps out with quite a few different Zu speakers about 6 years ago and always found the Audion more in the Audio Note school of rather lovely sounding but not particularly transparent nor immediate in nature in comparison with the OTL amps and preamps and even some other SET amps. Has Audion changed their approach to sound or is it just maybe subjectively we are not hearing these amps the same way?
Not my experience at all with Audion, so am not sure of any obvious explanation. They for me are uniquely fast and transparent sounding, but I didn't use stock tubes. I've gone head to head between Audion and Audio Note and I would not put them even in adjacent sounds; completely different. I first had a Golden Night integrated 300B SET about 20 years ago. Power tubes could be chosen for old school or new school sound. I ran the KR 300B -- big difference over the stock Shuguang cheapies or something like a Sophia/TJ mesh plate. Same was true for my Golden Dream monoblocks. The 845 Black Shadow is highly affected by its tube complement.
There's nothing in my over 20 years experience with Audion SET and push-pull triodes that aligns with your characterization as being Audio Note-like, or not being transparent or immediate. Speed, transparency and immediacy are the prime characteristics that have set Audion SET apart from the rest, for me. So I don't have an explanation for our differences, other than possibly the tubes. Zu speakers have those characteristics too, so maybe Audion SET character is more apparent and leveraged in a crossoverless speaker.
I did have my various Audion amps over time mildly modified, primarily recapping the power supply with electrolytics difference from stock, which further improved speed, immediacy and bass definition. But these were improvements furthering basic traits already present. Also, Audion has moved their sound further in this desired direction by updating models with better grounding, build quality, parts quality, and they have been winding their own transformers for the past decade or so.
There might be other variables. There are a lot of speaker cables that are not helpful to the sound of relatively low power SET. Many of the more complex cables smear and reduce immediacy. I view speaker cables as sort of fixed parametric equalizers. Few sound objective. I use Zu cabling because it is objective sounding and transmitting of the Audion qualities I cite. I'll put it another way: I auditioned SET amps for a dozen years, 1987-1999 and walked away every time until I happened upon Audion at the end of that period. Audion was the first SET I heard that delivered immediacy, transparency, agility and speed. If you try Audion SET, get KR power tubes. Forget Takatsuki, WE, Sophia, etc. BTW, same qualities are extant in their preamps, but again, upgrade from stock tubes.
Phil