Here's something I wrote almost 15 years ago. I'm open to correction on what I stated then:
When I was first introduced (formally) to the history and grammar of the English language, I was taught the outmoded symbol-referent model. I have since abandoned it in favor of the signifier, signified, and referent paradigm. Now I make that observation in order to stress that I am not saying the qualities of the "terms" (θεὸς or διάβολος) are emphasized by the preverbal anarthrous PNs; to the contrary, it seems that the qualities of the referent (or grammatical subject) are stressed in John 6:70 and 8:44. So the respective terms or signifers ("slanderer" and "liar") apparently stress qualities of the subjects (Judas and Satan respectively) discussed by John. In John 1:1c, therefore, it might be the case that θεὸς primarily serves to delineate the qualities of the Λόγος (the referent or grammatical subject) as opposed to depicting his identity. This is not the same as claiming that the quality of θεὸς is emphasized over against "the qualities and all" of the term in this fateful passage. As the KIT appendix says: "It [θεὸς] merely expresses a certain quality about the Word [Λόγος] . . ."
But how does one prove that a construction is qualitative (with very little if any emphasis on indefiniteness) rather than definite or qualitative-indefinite or indefinite-qualitative? Work from usage, grammar [syntax], and context. Hence, I may not agree with Harner or Dixon's statistics for qualitativeness in John (Harner said that about 80% and Dixon wrote that 94% of preverbal anarthrous PNs in the Fourth Gospel are qualitative, I believe). I may question some of their examples and the inferences they draw from the data, but 6:70 and 8:44 evidently stand as legitimate examples of places where the qualitity of the subjects are stressed (as does 1:1c). But are these examples of pure qualitativeness? Maybe we don't have to go that far.
Now as far as fronting for emphasis is concerned, Wallace gives John 5:10 as an example of a preverbal anarthrous qualitative PN. Frankly, this verse is highly debatable. Nevertheless, it's possible that the reason why there's a fronted PN could be for the purpose of emphasizing the qualities of a literary or grammatical subject. If the Jews were arguing about the "kind" of day on which the man was healed, then John's use of Σάββατόν in 5:10 could be primarily qualitative in nature. Granted, this conclusion cannot be reached by the syntactical construction alone; however, I am simply pointing out that anarthousness might be used to focus on the quality of the subject delineated by the signifier in question. But we must determine such qualitativeness or lack thereof from the context. Even in English, we may front nouns to emphasize qualties.