The sub is always on when plugged in with the control pod hooked up (not sure how it is when it is not hooked up)
The DIN takes the power from the sub to the control pod where it powers the opamps for the preamp and headphone output) The singal is then sent back to the DIN and the sub for the speakers.
The fuse on the power board inside the amp.
If you do not know anything about electronics, don't attempt this. Find a friend/someone who does and is used to working on PCBs.
You can get an idea of what you are dealing with here http://forums.klipsch.com/forums/t/102240.aspx
If you know what you are doing, you can open the control pod up, remove the two screws that hold the board in and remove that (have to push it sideways of a sort and pull up lightly on the cords to remove it. The headphone and input jack sorta hold it in.
http://forums.klipsch.com/forums/t/87492.aspx You want to check the yellow/blk and the yellow wires for +-15V. If you are not getting it, the sub is not sending power.
If it is getting voltage, then check the voltage at the opamps. (careful, you can easily short out the opamp)
You will see two opamps, of the upper one (closest to the cables) will be upper left corner (there should be a dot on the opamp to show you) that is audio output from the opamp. Pin 4 is the one on that same left side but the bottom pin. That is the -Voltage pin. The upper right pin across from pin 1 is pin 8. That is +Voltage pin. If you need better explanation, look for the schematic of NJM5532 online. Careful not to short anything out when you turn the power on and test with the meter leads.
You can also open the sub up and check for voltage there at the DIN connection instead of the control pod and also voltage in the amp in general. You must know what you are doing or you can be hurt. You are dealing with house voltages in some areas of the amp. You will need to disconnect the sub speakers in order to remove the amp section. Make note where the wires go on the speakers.
Hope it helps a little. Don't kill youself.