If just one STB is not working, no reason for tech.
They should just send you a new one.
No reason for any other testing.
To be 100% sure, take the one that doesn't work and try it in place of the one that does. If still doesn't come up, then definately bad.
You can try contacting them via twitter @verizonsupport.
Just make sure you let them know it is just the one STB that is bad.