I get it but I think you're missing the severity of the situation. It wasn't bad sound. It wasn't a crappy monitor mix. It was me and a basher drummer a ways behind the mains and NO sound on stage. Not a single wedge anywhere on stage. No, I couldn't hear the lead over the drums. Walking out is indefensible in absolute/theoretical/principal terms, but I made a calculated decision that I would rather suffer whatever damage to my reputation might come (turns out that's none) over enduring this torturous disaster. Really it was so bad, and so unnecessarily and egregiously so, I lost my mind.
As far as being an "amateur" move, I have made my living as a musician in the past. I don't anymore. Although I only play for a minimum dollar amount as a matter of principal, these days I only do it for fun. This gig was definitely not fun.
I'm not trying to judge, be condencending, or justify my opinion here, but I do grasp the severity of the situation. It was a bad mix. It was compounded by the fact you were doing a sub job and weren't familiar with the music, players, or style. Keep in mind you weren't the only one on stage dealing with adverse conditions that night, the entire band was as well.
You got frustrated, angry, then rage quit, walking out on the band mid show.
Being professional is gauged by many perimeters, not only how many gigs you play or if you make a living doing so. I have worked with many flaky "artists" in my past, some more successful than others. Most have been workaholics and incredibly passionate about their craft though. Believe me, I have seen some pretty big and childish fits thrown by them as well. But in most cases, it's their show and reputation on the line, not mine.
By you walking out you damaged the bands reputation as well, and that would kill your chances of working with any A bands where I live. if I read right that you left your bass and gear, you are very lucky that it didn't get "lost" or "stolen" after the gig. I can't imagine leaving my equipment at a gig.
What I can't imagine even more is after the fact you flaked on them, they asked you to join the band. Wow. That says it all right there for me.
Like I said, I'm not judging you or your abilities, but being taking a job as a hired gun, and walking mid gig because of a bad mix screams amateur to me. A professional simply does not act that way. In fact, you would have never known anything was wrong watching a professional in the same setting. That's maybe my perspective, but it seems to hold water in the business and players I work with.
Edited to say I no longer play professionally either, where I make a living based solely on playing, but I do work with pros that keep the same mindset. Many have worked with national acts as I did. Once that level of professionalism is engrained in you, it's hard to work any other way, well at least it is for me. I've been pretty frustrated at shows in my past as well, I'm just lucky I have a slow burn fuse.