TBH it's pretty rare in my experience to get a full-frontal slagging off, so probably best to chalk it up to experience & move on. Most guits are pretty relaxed at the jams I go to, so long as you stay in the pocket, but there are a few Rock Stars around who find it surprising that you're not Tommy Shannon & can't pick up the key immediately from the appalling racket they consider their 'core tone'. You soon get to know who they are. That said... in your OP you say that you "know what a 12 bar I, IV, V progression is and can play a simple bass line to it". I wonder if perhaps the guit found your lines too simple and/or generic. The fact that he didn't give you a heads up on the third number suggests that his back was up by then. If you were hitting R-3-5-6-b7-6-5-3 a lot, it may not have had the variation that he was expecting. Sometimes that's all the song needs... But then again, sometimes you need to mix it up... Any soundclips you'd be prepared to share? Pete.