Here's the story. I recently purchased a used SWR Grand Prix preamp from a business on Ebay. It was listed in working condition. When I hooked it up to my rig (Carvin DCM1000 and Eden 410xlt) and powered everything on, I got nothing but static. However, when I plugged my bass straight into the power amp, things went alright. Furthermore, when I hooked up my friend's amp head to the power amp through the preamp out, everything worked. Is it possible the tube in my preamp is bad? Any other ideas? Also, how can I tell when my tube does go bad? Will the unit just fail to power on? thanks!