big > deep; loud; powerful base > base; bass
3 And on the rock the waves, breaking aloft, 4 A solemn mean to them measured;
mean > middle part, baritone, tenor measured > apportioned, meted out
5 The whiles sweet Zephyr loud whistled
The whiles > Meanwhile loud > loudly (if applied to his whistling); loud (if applied to Zephyr)
6 His treble, a strange kind of harmony; 7 Which Guyon's senses softly tickled, 8 That he the boatman bade row easily,
That > [So that] easily > without hurry
9 And let him hear some part of their rare melody.
rare > unusual, strange
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