Unwanted Distortion?

Hi. I have a mid 70s Vox Escort Bass50 amp. The problem is when I turn the volume up more than about 30% I get terrible distortion and a punchy sound when I hit the strings on my bass. I changed the output trannys which helped a little but the distortion is still there. I also checked the circuit board and re-soldered some bits and pieces but that made no difference. Vox UK haven't a clue. They told me they took over the Vox empire mid 90s and know nothing about the old amps. Waste of time that was!!. Anyway, is there anyone out there that can help me please. Thank you.