OK - so I'm stumped. Marshall recently serviced this amp wot I've just bought. Marshall changed the bias caps (pretty usual, I know). I thought they'd 'forgotten' to put the bias pot back, but I now I see that some amps of this period didn't have a bias pot at all... So I guess Marshall is correct but how do I bias this puppy? And is it considered very bad form to add a bias pot in the same position that later plexis have them?