In my experience of playing these sort of shows when i was gigging this sounds nothing out of the ordinary. its aggravating as hell but its just the way it goes really.
I always found it was generally the headline band that lent out the majority of the kit i.e drums (except breakables), and amps/cabs.
best bet is just to take your own head if youve got one and ask to run through the cab, if not wouldnt worry, be polite and grateful and generally people are happy to lend kit aslong as you treat it right and dont assume that youll be using it regardless.
A lot of the time wed end up taking full backline and leaving what we didnt need backstage/ in the corner of the club because the promoter was useless.
wouldnt worry too much 90% of the time as it'll all change on the night in terms of gear lending, and being a bass player worst case scenario youll be straight through the PA (which will either lead to an awful night for you or a fantastic sound depending upon the soundguy and rig etc).
I'ts all part of the fun of being in an unsigned band, Embrace it generally (IMO) most promoters havent got a clue, youll suss out the good venues and good promoters and end up cutting out the useless ones.