What is an AD-100? And if it has 8 kt88's why don't they call it an AD-200?
How is the customer using it? Are they really hitting the grids hard? What sort of speakers are plugged in and have there been any reports of sound cutting out when in use?
The last amp I built with big bottles over 450Vp blew through two pair before I landed a pair that would survive. I don't think they make 'em like they use to.
Have you clipped your meter on the grid pins to read bias and slapped the amp around a bit to see if it might be intermittent with vibration?