What’s wrong with staying in Ultralinear? You should prefer one mode over the other. I use Triode in my setup, Dynaudio 1.8’s which are also a 2 way + passive radiator small tower. They are probably your polar opposite in terms of voicing. I don’t really worry that it doesn’t sound as good in UL. The differences that you are describing are exactly what you should hear when switching from one mode to the other, and the one that sounds better will depend upon the speaker. Clearly the extra dynamics of UL work well w/ the 1.5’s. I think if you liked both modes equally it wouldn’t be as good of a match. You can try the KT120’s and see what happens but I doubt it will change your preference towards triode.
Actually for not much more than an Octet of KT120’s you can probably find the 1.8 mk2’s. Then just think of that button as the Dynaudio/Thiel switch.