You shouldn't have the sub all the way up like that. In most systems, that technically is not a volume knob for the sub--it is a Gain control for the Amp inside the sub-system. You've probably been overdriving the transistor and/or the coil in the amp--not to mention possibly the voice coil in the speaker. Unless it's a short in the cable, plug, port, or an RF/EMF problem--your going to need to replace it eventually.
In a lot of these systems, this "volume" knob is actually for attenuating the sub's amp to your soundcard, receiver, tv, or whatever source device you are sending to it. On some, you will have a "Bass" knob that serves as an attenuator and a volume knob that is the output to the whole system, or just the satellites. You should set the source to the typical volume level you expect to use and THEN use these knobs to adjust the output(s) of the speaker(s) to get clear output and the proper saturation at that level and then leave it alone. From that point on, you should use the fader or EQ/Tone adjustments of the source to change the output. This way you will avoid all possibility of overdriving the sub-system's amp or speaker components.
If it ain't broke don't try to fix it! But, hell if you can tweak that bastard just a little bit more....hehehe.