Important to know your instrument and equipment provided in the jamming studio. You may need to ask the studio owner to help you familiarise with the house equipments.
1) Singer know how to hold and use the mic, better volume can be achieve before feedback by this.
2) PA system should be working satisfactory (there is good chance a jam studio speaker horn is blown, due to overload by abuse or screamo's)
3) Instrument amps should be tuned properly and strictly used for it's purpose (eg. do not plug a bass to guitar amp)
4) If you are practising to mp3 backing, ask the studio operator first to help patch you in properly to the PA. (most will help when asked, courtesy to inform if you need to patch in any other things too)
5) All above adhere to, lastly follow the golden rule: QUALITY OVER QUANTITY.
(meaning, as long as comfortable listening level achieved, should be enough. If any member in band want to be a prima donna and wack loud loud, kick him out)
Remember, studio operators are human too. As long if you respect and ask nicely, you will most probably recieve it if its reasonable. Alway remember, treat equipments properly, for benefit of all.
If studio operators are cold and unreasonable, just go find another studio. There are many studios these days to choose from.
I am a jamming studio owner myself.