Good points; you push me to my limit.
Ultimately we are just talking about words, our own constructions. Joy does not arise FROM positive nihilism; it IS the positive emotional construction the positive nihilist puts on the realization that he is a grown-up who must and may take responsibility for the world HE (and his fellows) creates, i.e, their culturally constituted Reality. They must even take responsibility for their creation of God and for their having, I guess since the Enlightenment, killed Him (with the decline of theocracies in the West). Negative nihilists are those who find only misery in the loss of their parent. Their gloom results from their inability to grow up.
These are discriminations I am making; they are my constructions. Moreover that's the case with the ranking distinctions we are making between gloom and joy. Ultimately they are delusions, albeit real delusions. They arise as yins and yangs in a world based on false oppositions. Reality is experienced in terms of unorganized immediate sensations which are organized in terms of complementary contrasts such as good-bad, beautiful-ugly, up-down, true-false, etc. even the "real" and "delusional" that I am using now. The immediate sensations are "real" and their meaning, e.g., good or bad, is artificial and with respect to (or with reference to) the guiding standards (cultural constructions) of good and bad, or beautiful and ugly, etc., etc.
Finally, one can see, after some effort (i.e., meditation) that there is no subject of such experiences. One feels compelled to create and maintain an AGENT of experiences. But the fundamental instruction of Hinduism to the yogi is Tat tvam asi (Thou art that). This tells him that what he is seeing, hearing, thinking, feeling, etc. is not HAPPENING TO HIM, to a distinct self INSIDE of him, but that "his experiences" ARE him. My use of the phrase "his experiences" is not a contradiction/denial of the principle tat tvam asi; it is a linguistic convention that simply ignores it.
There is no real differences between good and bad or joy and gloom. Both are reality or what is sometimes called "Buddha Mind", the liberated consciousness in which all is ultimately non-discriminated (where distinctions may be useful but remain ultimately unreal). The problem is we do not live in Ultimate Reality. Actually we do but we do not realize it; we feel like we are living in the Relative Reality of our construction. Our task is either to create better relative realities, or "awaken" like the Buddha admonishes us to the Ultimate Reality where experience is not interpreted dualistically.
By the way, Anti-dualism is finally also part of a meta-dualistic scheme where it is understand as something-against-dualism itself. Only silence speaks the truth.