My power amp ARC VT 100 keeps blowing tube

First off, I never use KT88's in ARC products. That being said, I'm guessing you have a problem underneath V16 tube socket, or the tube socket itself is bad. The problem is not necessarily setting the bias on VT100's, the issue is setting up the balance on the front end of that amp. It is not an easy thing to do. The input tubes must be matched and I only use ARC tubes. Replacing the input tubes without setting it up properly can shift the D.C. operating points thereby raising distortion and potentially causing issues.

Sounds like a trip to the service center to me.

If you are in or near Cali. I can help you....