What causes 1 speaker to distort unexpectedly?

I have Totem Winds and the left speaker, only, will crackle and distort short bass blasts with the bass speaker undulating wildly for a few seconds. The mid and tweeter seem to have a hiss sound on this speaker at the same time. This problem just started today and has gotten worse. Everything is running off space-tech tube equipment, but this has never happened before, and as I said it is only happening in one speaker column.
Jc, might it be possible that the problem is stemming from upstream? Possibly at the amplification level?
Swap your left & right speaker leads, and see if the problem stays in the same speaker or follows the speaker cables. If it follows the speaker cables, then it's upstream of the speakers. Next switch the left and right inputs on your amplifier, and see whether the problem stays where it is or switches channels. By following this approach, you can isolate which component the problem lies in. My first guess would be that you have a bad tube somewhere.

sounds like a bad tube.Isolate as Duke suggests.
Duke hit it on the head. Sean
Thank you for the excellent suggestions. If it is a bad tibe how do I identify the bad tube so that I can order the appropriate replacement?

Many thanks again.