I bought a GK Backline 600 amp a few years back, but have had to put it into storage due to moving issues. I got it back within the last few months and haven't played a lot on it. When I turned on the amp, it seemed to take a bit of time "warming up," with the crackle sounds adjusting the volume. Now, I can't seem to get sound out of it. I tried plugging the cabinet into both ports on the back, but still no sound. I know my bass isn't the issue, as I just recently played it through another amp combo, and I'm pretty sure the cabinet is just fine. My question (or questions) is what could be possibly wrong with the amp? It still lights up. Also, if I can't fix it myself, would it be even worth it to take to a shop, or just buy another amp head? Thoughts?