What tubes are in it now? Appearantly you have not tested them. It appears that you don't want to use the phono stage. If that assumption is true, you can see if the phono tubes are stronger (which they probably are). If that's true, just switch them.
If you can't find out which tubes do what, just swap the left and right pairs-WHILE THE UNIT IS OFF. This might be all you need to retube your unit.
If you can't find out which tubes do what, just swap the left and right pairs-WHILE THE UNIT IS OFF. This might be all you need to retube your unit.