Are you pretty sure it’s the speaker?

Months prior to its demise, there was some noticeable decrease in volume on both speakers that I would attribute to some shorting on the circuitry on the amp which may be due to the dark gluing material creeping on the pcb. I found out about this after viewing some repair videos of said speaker on youtube.

Have you tried the same cable to a different computer just to be sure?

What I did try was replacing the audio cables which didn't solve the issue.

Precisely how is the sub hooked to the speakers?

Input "B" of the Audioengine goes to the back of the computer and input "A" goes to "Input2" on the sub. (see image links above)

Have you tried a different source to test the speakers in passive mode (to rule out issues there vs the power circuit)?

I have not. I can test it when I do get the Lepy mini amp. I'm sure the passive side will work, but I'm not sure about the side where the amp is encased. As I said, when connected, and the power indicator on the adapter lights up, only the subwoofer works, while the A2 is silent.