In my case, I liked the KT90's in the mini's. They had more depth that the small transformers seemed to lack. In the Cary amp I owned, I switched between the Winged C and the EH KT88's, but the mini mono's just needed more for my setup. The mini mono's were very good and a steal for the money. The mid mono's are a whole different level with the large transformers and seem to tube roll much better for me. The KT 90's were just too much or fat in the mid mono's. Besides the sound, tube amps are fantastic to fine tune your sound to your particular tastes and the reason I love tubes.
Enjoy!
Enjoy!