Halliday & Matthiessen (2014: 420-1):
The items serving as Premodifiers are adverbs belonging to one of three types — polarity (not), comparison (more, less; as, so) and intensification. The classes of adverbs that can
serve as Premodifier are thus considerably more restricted than the classes that can serve as Head; they correspond largely to adverbs from the degree class in Table 6-18. Those of intensification indicate higher or lower intensity; they are either general intensifiers that are interpersonally neutral (very, much, quite, really, completely, totally, utterly; rather, fairly, pretty,; almost, nearly), including the interrogative adverb how, or specific ones that derive from some interpersonally significant scale (amazingly, astonishingly, awfully, desperately, eminently, extraordinarily, horribly, incredibly, perfectly, terribly, terrifically, unbelievably, wonderfully).