I hate to say it, but now I think maybe I like my amp in ultralinear mode versus triode

@immatthewj If the UL taps are set up properly, the amp should have less distortion in UL mode. FWIW, this was well understood in the 1950s when UL was first patented. Distortion obscures detail; so if the output transformers are properly designed it should be no surprise that the UL mode sounds better.

A secondary aspect of this is power tubes in UL mode are easier to drive than the same power tubes in triode mode. So the amp very likely is also making less distortion on this account too.

I readily admit to how illiterate I am when it comes to electrical. Actually I had always thought that the one was a synonym for the other. I am in need of an education. Can you put into layman’s terms the difference between the three? If layman’s terms just won’t work because the subject matter is just too complex and it would take writing a book, I understand. I can live with just knowing that today I liked UL better today.

@immatthewj I’m no circuit analyst or electrical engineer, so I’m possibly not the guy you need to hear this from. But I’m a computer programmer who often has to explain things to people with a tenuous grasp of coding, so I’ll give it a crack lol. Based on my limited understanding, what differentiates a tetrode / pentode (call these both "pentode" from here on out, as the 5th element is not relevant to this discussion) from a triode is that the former have a screen grid that triodes don’t. The screen grid is positioned between the control/signal gird and the plate (the biggest metal part you see in a tube). You can sometimes spot it through the plate holes - its coiled wire is spaced out much more than the tighter control grid coils.

When you run these amps in "triode" mode, this screen is tied directly to the plate, and basically acts like it isn’t there (mostly). When you run them in "pentode" or UL mode, the screens are wired to the output transformers, but these 2 modes differ in exactly HOW they are wired to the transformer. Basically, UL is more cleverly done so that its distortion levels are much closer to triode mode than pentode (lower is better) and its power output is much closer to pentode mode than triode (higher is better). So you really do get "the best of both worlds", sort of. The output transformers do need to be designed with UL mode in mind for this to work. But frankly, it would be very odd to encounter an amplifier with only a "pentode" mode instead of UL (UL mode was devised in the 1930s and became common since the 1940s, e.g. the "Williamson" amplifier - incidentally, every time I hear of that "Wilsenton" Chinese amp brand I think that must be an Engrish-mangling of the name "Williamson"!). And those which offer switchable triode vs. "pentode" modes should also be rare - in those cases, you should probably just stick to triode. The only meaningful choice is triode versus UL. And in my opinion, borne out by when I’ve tried this - if you’ve got tetrode / pentode tubes running in your amps, you really want to be running UL mode. That’s the best use of their design elements.

True power triode tubes, DHT, and SingleEnded (these 3 often go together but not always!) are other balls of wax - I’ve heard them a couple times and don’t think they are for me. At least, I haven’t been won over yet. I listen loud and feel like I benefit too much from the extra power of UL mode. Give me tubes which slam, or give me solid state! (that’s better than death, I guess)

Of course, a pentode tube strapped for triode has two physical elements within the tube that are shorted, so not functional, which is quite different than a tube designed as a triode, so it is not surprising. For a triode strapped pentode, power output will also be roughly half, at a higher output impedance, so required speaker damping and epdr probably come into play as well.

It says right in your owners manual that the designer, Dennis Had, preferred the amp in triode, for whatever that is worth.

With the tube amps I’ve owned that can switch between the two modes of operation I always ended up preferring Ultralinear mode,mirroring your findings of better imaging definition,slightly forward staging & more "meat"on the bones in the bass & upper bass...I do however believe this is speaker dependent..All of the speakers I had at the time were already naturally warm sounding(Harbeth M30.1,Fritz Carbon 7SEMk.II & Sonus Faber Lumina III),negating the benifits Triode mode provides...With a speaker that has some tendency towards a more lit up-forward tone(Triangle-Kef-Klipsch or anything with metal drivers) I think Triode mode might be the way to go...