I've been an SVT and Ampeg user for years, mostly all tube heads and SVT's are still my favorite sounding heads of all time. That being said if I were to ever consider getting a lightweight class D head at least at this time and admittedly only from listening to different videos it would probably be a Mesa D-800, they sound very close to an SVT to me or at least the person who did the video made it sound like one, @DJ Bebop did a great video with his triple pickup '62 P bass on youtube and played through one of the Mesa 800 heads (don't know the difference) and it sounded great, maybe he'll post it here.
I also agree sticking a preamp tube in a SS head doesn't do much at least for me, it would be nice to have but I have never experienced a hybrid head that sounded anything like an all tube head, the heads I've tried just lost crunch and clarity with the tube engaged but didn't get any of the all encompassing bottom you would expect with an all tube head.