To me, viability of a small amp like a Blues Jr. in a live setting depends upon the following variables:
- drummer
- size and type of room/venue
- style(s) of music
- size of act/band/ensemble and type of instrumentation
- your needs and relationship w/ clean headroom; what your idea of clean tone is
- how well you hear combo of overall mix/your signal
- stability or diversity of volume levels/dynamics
- whether you mic' the amp
- whether you can (and/or want to) hear amp signal in monitor(s)
Single small amps have almost never * been a good fit for me personally in live settings, but I absolutely love running a pair of small amps.
* I don't have a Blues Jr., but I have an early Pro Jr. Great amp, I love to crank it in the studio for lap steel tracks. The only room I've played fairly regularly in the past where I learned to always bring the Pro Jr. was Fat Matt's Rib Shack in Atlanta; small stage, small, odd room. First time I played that room, I brought an AC30, which was.... beyond ridiculous. Also, I always played the upstairs (larger) stage at Tootsie's in Nashville; if I ever played the downstairs room, I'd bring the Pro Jr., as nothing else I own would even fit on that tiny matchbox of a stage. However, 99.9% of every other live situation I've lived through, I gotta have more amp than that.