markbass vintage preamp

After two years of moderate use (two times a week) it seem to me that it lost some of its distortion. I suspect a failing tube. What would be the symptoms of blown tube on this preamp?

I have made a fair share of research into bass preamps in the last month, trying to find a fitting preamp (pedal) for my new passive G&L Kiloton. I do like how it sounds at home at my amp, but I play into a DI at church, and I wish to have a few EQ/Boost options the way I do on my 6-string...