Understood - things like that can get tricky even for experienced folks....
If you cannot control the base then the control (potentiometer) itself would be the starting point.
Google "how to repair a potentiometer" and take a look at some of the websites that are presented.
Here is a good exampe but not for your speaker per se:
https://www.youtube.com/watch?v=AXyO7VjaW_4
You should be able to get a sense of what is involved and what may be required for a fix. Could be as simple as a short (two electrical wires touching when they should not be. Maybe loosened up and vibrated into contract by the high volume.
Your photograph shows six screws on the back panel. If you are up for it you can unscrew them and gently lift off the panel. There may be components attached to the back panel so try not to pull it out too far and disconnect something. Use a bright flashlight to look around. and compare what you find to the various links on the internet. Look for burned insulation, bare wires touching something else, etc., etc..
Take some photographs of anything that does not seem correct.
Most important thing is that you unplug everything connected to the speaker especially the power. And be careful with the entire process. Hopefully you have a friend or family member with some knowledge that can help as well.
If you are really lucky a trip to Radio Shack (or similar) store will provide a replacement part (potentiometer perhaps) that you simply reinstall and plug in the wires again.