I have not yet listened to an Ls100. I have a PrimaLuna Evo400 Preamp and the matching power amp. Although there is certainly a smoothness to the sound, I wouldn't go anywhere near a description like "tubey". To say it's "syrupy" seems a wild exaggeration. To be honest, I think the PL gear is closer to a SS sound than more traditionally sounding tubes.
I'm no expert, but it would seem to me that comparing two tube preamps would best be done using driving a known neutral power amp. Only then can you truly appreciate what the tubes/design are doing for you.
I'm actually considering trying an Ls100 to get MORE of a tube sound, based on what I've read.
Cheers,
Robert