Common wisdom is that the driver tubes may need damping but not the power tubes. Kevin Deal advises stongly against tapping on tubes as this could make exaserbate (sp?) the problem by loosening other components inside the tube. I bought HAL-Os and they seem to work well and they are not expensive but hi-tech at the same time.
http://herbiesaudiolab.home.att.net
http://herbiesaudiolab.home.att.net