If you connect your bass to the Associated Press (AP) news organization you might get some free publicity, but you'll definitely get a better sound by using a bass amp.
Seriously, your best bet in this situation is to put only vocals thru the PA. The band must control its volume carefully. The short answer for that is for everyone to TURN IT DOWN until the vocals are out front.
Don't put your PA speakers directly on the floor. Raise them up at least a foot or more; get a couple cinder blocks from the hardware store, or if the speakers are small, you can put them on chairs.
Finally, if the room is all hard surfaces and sharp corners, do something about that if you can. Put acoustic foam or drapes in the corners to soak up some of the reflected sound. If you can hang a tapestry, drape, or other sound-absorbent material across the ceiling that will help too. To find the best places to do this, listen for reflections in the EMPTY room when you clap your hands loudly or pop a baloon in different places in the room.
Don't cover every single surface with sound-dampening material; a room that is too dead is problematic in different ways than a room that is too live.
Good luck.