I've never heard the Avalon U5, but it looks like a very nice preamp targeted at instruments. You may want to get another one since you're happy with the sound.
I've used several preamps for bass and guitar with good to really good results (Neve, ISA One, TB12, ADL600, VMP-2). My favorites have have been ART PowerPlant (elec gtr), J-Station (elec and bass), ISA One (for bass) and Amplifire (elec gtr) for direct. Using a good mic and any of the above preamps worked well for acoustics. The J-Station in particular is still my go-to for direct bass guitar while the Amplifire see most of the recording duties for elec gtr. Good results every-time.