Right Channel is Wrong...

Could use some help, here. I know what sounds good but when it comes to problems, I am a bit of a stupe. :-(

With some increasing frequency I have been experiencing loss of signal from the right channel of my system, especially when using my old Sony analog tuner. Out of the blue, I have virtually nothing from the right channel except a very faint signal from the tweeter in my PSB Stratus Goldi's. After shutting down the system, letting it sit for about five minutes, and then restarting, all is well, until the next time. When powering down (first the amp, then the preamp, finally the CD and tuner), I notice a crackling from the woofer and an extrusion of the cone when I power down the preamp. I have bypassed the preamp by running the tuner directly to the amp using the variable outputs and have no problems. As such, I suspect a problem with the preamp. Any ideas of the cause and do I risk speaker damage with continued use before the trouble is "nailed down"?
David you can & should always turn off the PA first in order to avoid the pre. etc. introducing damaging transients upon power down. Conversely, the PA should always be the last component that you power up during a turn-on sequence. Last on; first off.
If the pre is tubed then check tubes & swap them around if possible (power MUST be off) but if solid state then it's likely in need of service. You might want to poke around inside looking for loose / dirty connections if you're familiar enough to do so.