Aha! A 'head tune'. They're always the better compositions...written by the superior subconscious composer.
The neapolitan is, in classical harmony, the bII maj chord of a minor key. It's an archaic, bluesy substitute for the V chord. In this case, the bass is playing the leading tone [G#] of the Am tonality in the bottom of the Bb chord [ the dom7 of Bb.....also the b5 of the 'ghost' E7].....so the chord could be said to function as an E7b5 [root b5] lending a whole-tonie kind of moment [a more jazzy construction] on the V7. [if you played a solo over the changes, that would be a great place for a Steve Vai 'out' moment, whole tone phrase, eg] And the G# chromatically resolves to the tonal root nicely. Strong shit.
Any way you look at it, it just struck my ear hard, and pleasantly. I don't hear that kind of musical finesse in most radio rock. Unlass it's like, Barry Manilow, who rips a lot of his tunes from 600 year old classical pieces, but has no edge whatever. :^)
So, like, did your mom play a lot of classical stuff in the house when you were a tyke? You got ears. Listen to that music playing in your head. And render it faithfully...
In other words...go with this muse!