So.....after 2.5 years of fussing with this nonsense, I believe I've discerned the difference between series and parallel wiring and I may be a bit better at understanding impedance. That said, for anyone following, here's the latest:
I've series-wired my external speakers. It took me 1.5 hours to get it right but everything sounds good.
Internal Pink and Violet/ Purple speaker (+) wires are connected to E4 right and left taps. Internal (-) wire is connected to E1 tap.
Right side EXTERNAL negative (-) wire is connected to E1 tap. Right side EXTERNAL (+) wire is connected to E6 tap. NOTE: I'm only using two wires - one negative and one positive).
External (-) wire from the E1 tap is connected to the (-) end of the RIGHT side external speaker.
External (+) wire from the E6 tap is connected to the LEFT side external speaker.
A separate wire is connected to the (+) end of the RIGHT side external speaker. I used the SAME wire and connected it to the (-) end of the LEFT side external speaker.
I'm still using the two 8-ohm Polk speaker so I should be at 16-ohm impedance which this amp should tolerate nicely.
Of course, since the amp bench tested fine when it was sent back to A&B Jukebox for the third time, I'm guessing when I ran (+) and (-) wires from both external speakers back to the right and left side of the internal tap board on my previous attempt(s), it resulted in some kind of low (4-ohm or less) impedance issue. As mentioned, the speakers would sound great for a day or a week and then begin to permanently distort.
A&B Jukebox thought a weak weld somewhere in the amp may have resulted in overheating/ distortion but again, the amp was on the bench playing for 10-days and it wouldn't distort.
Interestingly, on this attempt, to series-wire the external speakers, I actually PARALLEL wired them. I'm guessing I dropped impedance on the 8-ohm speakers to 4-ohms (or less). Obviously, I didn't leave the speakers wired this way but I thought they sounded better wired in parallel than they do in series.
If you're able to make sense out of all of this, God Bless You! As with the case of my Detroit Lions, I'm keeping my fingers crossed!