I have both right now. I have the a90 as well. I've only had the soloist a week, but both have their strengths. 789 for sure is cleaner and more analytical while the burson is a little warmer. Love the soloist so far with my Arya v2. The 789 can be a bit bright at times, but not fatiguing
The 789 I feel is quite wide, but without that much front-to-back depth. The dynamics could also be upped a tad.
I like the linear tonality of the 789; I am using Burson Classics in a Gustard P26 preamp fed to the 789 which is working like a microscope.
I can say that through this class A preamp, neither the Classics or the Vivids are completely transparent. Running the preamp at full volume (99) you lose the 'splishyness' of the very upper treble like cymbals and sibilance area is toned down. I like to run the preamp at 50% or less to get some of those details back (from my Soekris 1421 R2R DAC being at line level). However the soundstage does increase quite a bit with the Vivids being run at maximum volume. The Classics likewise make the stage more intimate and warm the sound considerably.
I anticipate this tonality would translate to the Soloist 3XP when I get around to ordering it.
Would also like to try some of these other opamps in the P26 (mugz, sparkos, staccato, orange etc) for a more honest sound. I think the staccatos might do it.