I like valve amps for bass guitar, and have had a few different ones. I've never gigged with them on double bass, but I've used them at rehearsals a few times where it was just what I had with me. If you're staying within the headroom of the amp, it can sound very good. The character of it varies depending on amp and pickup choice, but in general they're a little more obviously amp-y sounding than most solid state amps, whether in a warm, rounded way or more mid-forward.
I'm thinking I might bring my Ampeg PF-50T for a little gig I'm doing next month, a trio with drums and low volume electric guitar, so I'll report back if I do.