Volume is generally the reason there isn't PA support at these open jams. Competent players will match the amps volume with the drums and band. A drummer, 2 guitarists with amps and a bass player can be loud for a small bar. They don't need PA support.I don’t play professionally,I attend a few different open mics…and listen a lot ,,take note of what amp and cab configurations are used.and basses,and strings . At these open mics,,the bass is not running through the PA..which I find irritating,,but I guess the PA system that is brought by whomever is running the jam is not capable? At least this is what has been suggested to me. Anyway,it seems to me that a 4x10 cab seems to be heard much better than other cabinets. And I have heard guys running single 15’s,,2x10’s,and even a combo one 15 and a 2x10… so does this make sense to anyone here?
I ran a weekly blues jam for about 1.5 years, and have attended numerous jams in different cities. At a decent jam there will be one bass rig that all bass players will be using as they rotate through their turn to play. Some guys have active pickups with a hot signal, some basses are passive with lower output, some are passive with high output. It's difficult for a bass player to plug into an amp they're unfamiliar with and get their tone or volume dialed in the short time they have up there. I've seen everything from guys turning their volume knob down because they were timid, others trying to turn the volume on the amp way up, guys with active basses not engaging a -db switch etc... If a jam had PA support these issues would be mainly resolved by a good sound man. The majority of jams are in bars where the vocals get the PA, sometimes a kick drum or acoustic instrument and so forth. If you're doing open mics at a more amateur location, guys might be bringing in their own rigs, then you've got even more variables.
Making tone judgements about basses, amps and speaker size from listening at jams or open mics really won't offer you valuable info. If you frequent the same jam, then you can start gaining some useful info: you'll learn the particular amp being provided, you may become friends with the house bass player and maybe they spend more time with you on getting your sound up there. Has the bass player rolled back his volume or tone knob? Has the guy with the active bass boosted his mids/lows/highs by +15db? If the cab has a tweeter is the knob on the back fully open or turned down? Far too many variables.
A good 4x10" cab could be eq'd to sound muddy, bright or just right. Same with a 2x12", 1x15" etc...