80 watt Sunns are just 60 watt Sunns with solid state rectifiers, the audio tubes are the same. What hasn't been mentioned is that 60 watt Sunns also have a GZ34 rectifier tube, which is not present on the SS rectified amps.
If you have an 80 watt Sonaro it's probably a 1970, maybe a 1971 model. A picture of the back of the amp would tell a lot about it.
So, back to the early distortion issue.
Did the amp suddenly start getting nasty at 4, or did it gradually start getting dirty quicker? I know you mentioned on talkbass that a tech went through the amp; do you know or can you find out if the electrolytic filter capacitors were changed, and if so with what? The distortion situation could be a lot of things, including the tubes, but the filter caps are frequently the cause of that type of problem and the stock caps are a bit of a weak point using modern wall currents.