Relative nonrestrictive clause: "where they can debut unreleased and sought-after music"

Solution 1:

Structurally, it can't be a Noun Clause because the last words before it: "unannounced top DJ performances" already make up a Noun Phrase.

It can be either an Adverb Clause or, like what you strongly feel, an Adjective Clause.

But semantically, "where they can debut unreleased and sought-after music" doesn't really perform the function of an Adverb in that sentence.

This is why it's an Adjective Clause.

Solution 2:

where they can debut unreleased and sought-after music

is a restrictive Relative Clause that modifies the entire Noun Phrase

an artist’s haven for unannounced top DJ performances

This NP has haven as its head noun. Since haven can have a locative sense, it can be modified by a locative relative clause beginning with where (similarly, only NPs that can have temporal senses can be modified by relative clauses with when).

And, since it modifies the entire NP, it has to go after it, which means after the prepositional phrase at the end. That's all, really.