Which KT90, the Electro Harmonix? In the Rogue tube amps, EH KT90 has more treble energy while being leaner through the midrange and bass compared to Tung-Sol KT120. I think almost everyone prefers the KT120, unless you have a particularly dark system that needs the extra zing up top.
KT120 have excellent warmth, body, and bass impact. Great for rock. They have a bit of "sweetness" to them, but not as much as you'd expect from say EL34. You may or may not find preference with Gold Lion KT88, which have extra airiness and refinement to them, and slightly sweeter midrange, compared to KT120.