I have both an SFL-2 and a SF Line 2 connected to both SS and tube systems.
I find little to argue between them. In the Line 2 I found that NOS Amperex 6dj8 (orange globes), which are reasonably priced, worked best in my systems (both SS and tube). I am currently using 6h23eb (Russian) in the SFL-2 which works extremely well in tightening up a "soft tube" sound with an all tube system. Both of this tube choices also work using SS amps. You can tell instantly that either of these preamps are tube rather than good quality SS but they still sound tight enough for great listening.
I find little to argue between them. In the Line 2 I found that NOS Amperex 6dj8 (orange globes), which are reasonably priced, worked best in my systems (both SS and tube). I am currently using 6h23eb (Russian) in the SFL-2 which works extremely well in tightening up a "soft tube" sound with an all tube system. Both of this tube choices also work using SS amps. You can tell instantly that either of these preamps are tube rather than good quality SS but they still sound tight enough for great listening.