I have 834 Int so can't help.But I think they might have it shown in manual you can check online.Either that oir I am confusing it with CJ or another tube piece I have had.It would be easy to email there tech google Mitch Singerman who does all the warranty work for EAR in USA and ask him about tubes your using,if he'd try others etc.
Cheers
Chazz
Cheers
Chazz