There was no real "switch" to or from DHT DACs. LampizatOr DACs have always come with a variety of different tubes utilized. If you look at the brand history you'll see everything from 6sn7/vt99s, to 12au7, 6n6p, 4p1l, dhts, pentodes, and even some large and unusual indirectly heated tubes.
Each type of tube requires different kinds of design concerns and solutions. Here is a video of Lukasz discussing the tube decision for pentodes in Horizon; https://youtu.be/ha_ltmaGu20