An F1X is the ONLY way I've ever gotten an amplified sound that is true to the tone of my 1934 American Standard. It recreates both plucked and bowed tone accurately. I've used both Fishman piezo and Schertler electrostatic pickups. (BTW - The Schertler sounds much more natural to my ear.)
The F1X has two things going for it 1) 1Meg input impedance which is needed to keep all the tone from a piezo, and 2) a tube.
I actually prefer the F1X to the F2B for upright. I run it through my PLX and a Bergantino HT112 and hear the tone of my bass, only louder.
Before I got this rig together, I had been searching for about 15 years for a good amplified tone.
C-Ya........wayne