Can anyone tell me the correct bias settings for a Marshall TSL602? There is conflicting info out there.

Tthe TSL range have 3 pins under an inspection cover on the back. Some people say that the bias should be set to 40mv between the middle pin and the outer (total 80mv). Others say that it should be 80mv per side.

I’ve had the amp for 18 years and always have it set just short of 80mv, but I never seem to get much life out of my valves – probably half the 1,000 hours you’d expect. And the sound always seems to lack a bit of treble. Whenever I have it serviced, it always seems to come back set at around 80mv per side but it always seems to run a bit hot to me.

 Should I be lowering the bias?