Backert Labs Rhythm 1.4 preamplifier: New Stereo Times review on a great line-stage

In what ways do they part ways?  That’s really what I was asking. You seem hesitant to describe the critical differences. 

Hey soix,

I truly don't get what you don't get. Call it a draw between the two line-stages, in the ways I answered your question above. The "ways they part company" is totally dependent on the differences between the sonics of a 6SN7 vs 12AU7. 

When I use my all time favorite 6SN7 NOS TungSol black glass oval plates I experience a slightly rounded leading edge to the note, holographic imaging, sense of sound-space, where it was recorded, as much as the sound-stage of the music.

When I use one of my favorite NOS 12AU7, the Mazda Silver plate, the leading edge is more precise, there is more punch/dynamics, less warmth, but still great tonality.

The difference is in the tubes, not the preamps themselves. Both units disappear and let the signatures of the type of tube and different brands to be heard.

Don't know how to explain any better

Well, you finally described the sonic differences between the tubes.  That’s what I didn’t get and was looking for.