Checking the speaker is simple enough. Remove it. First inspect it for tears or anything loose- spider, dust cap, etc. Next, applying even pressure, move the cone up and down and see if you here anything rubbing. And/Or, hook the speaker up to another known working amp and see if it does the same thing. You shouldn't need to place tissue behind the cone if the speaker is working correctly. Conversely, you could hook a known good speaker up to the amp and see if the distortion is still there. Until you verify it is not a speaker problem, I wouldn't mess with anything else.