I've been using the same tubes in mine...yeah, they sound great in the MICA! What happened...did the head just completely lose power and shut down? That has happened to me on a couple of gigs. First time it happened, I thought it was the tubes, but it's not. I believe it has a tendency to overheat and then it just shuts down. Someone else said they had this issue and solved it by opening up the front of the head shell and also installing a fan in the back...that way there was better air flow and it didn't heat up so much. I need to give this a try because I've been hesitant to use the MICA lately on gigs...I'll bring my M4/RT2-50 setup because I know it's rock solid dependable. But I sure miss playing the MICA on gigs as it is one kick-ass amp!