On a gig a few days ago my Eden WT-400 seemed to be working fine, and then suddenly the signal coming out to the speaker just stopped. the DI still worked, though. i let it cool down and used it the following day and it worked fine. but today i was playing it on a lower volume and the signal cut out again! what could this be? could it be overheating? could it be that the preamp tube needs changing? any help would be most appreciated (I hope I don't need to buy a new head!). Thanks.