Playing a gig the other night, a little louder than usual, but not excessively so. Playing a passive bass through a 90s Hartke 3500, into a Hartke 1x15 Transporter cabinet. Tube and SS preamps were on about 3, master output around 5. About 20-30 minutes into the set, my sound got increasingly distorted over the span of about 5 minutes, and then cut out entirely. Luckily, the direct out was still working, so the sound guy just put my bass through the house PA for the rest of the set. I'm at work, and haven't had a chance to troubleshoot this. Could it be as simple as just the tube in the preamp burning out? Would that kill the sound coming from the SS preamp as well? And if that's the case, why would the direct out still work? Or did I blow something in the cab? Any input would be appreciated.