Problems with RD 100 Distortion

I've posted this question on a few other threads and can't seem to get an answer.
The distortion sounds real tinny and bad when I turn up the overdrive. The serial # is DP 09095 and chasis # 2100 RD CAN. It has GE 6L6GC tubes in it right now. They don't match because one is shorter than the other. I've had it for a few years and have simply used a distortion or overdrive pedal with it to make it sound good. Since I found this site I've become more interested in restoring it. PLEASE HELP!