I have now managed to try out the acidic modifier on skeins from the Eucalyptus gunnii bark dyebath. The skeins on the left were mordanted with alum & those on the right were unmordanted. The acidic modified samples are second from the top in each case.
It would appear that this species of eucalyptus follows the usual pattern of modifiers & the acidic modifier has lightened the shade. But it was worth doing the test just to make sure.