The correct polarity for the BT 7.1 speakers. The large terminal is negative and the smaller positive. On the Biketronics supplies wiring the black striped wires are negative.

On the BT Universal Line Levelers the Gray wires are for the left speaker and the White wires are for the right speaker. The striped wires are negative.
The Red RCA is for the left speaker and the White RCA is for right speaker.

 

I seem to have resolved my issues with the tinny sound from my amp and it looks like it was a polarity issue.
Just for the record, I did have the polarity correct with my initial install, but had a tinny sound, as if the treble setting was on high and the bass setting was on low.
I reversed the polarity on both speakers today and it sounds fuller and better. So I now have the polarity completely opposite from what the instructions say. Not sure why this works better for me, but it does sounds better.

I left the ground wire on the triple tree, it would only take a few minutes to run it back to the battery, but not sure if that will make any difference? I've heard the shorter the ground wire the better. Would grounding at the battery make any sound difference?
I have a feeling that in general the 2180 might lean a little towards a more treble sound anyway? (or at least the one I have does?)

 

Check these two out. Just ordered the Kenwood but both nice units for 119.00.
Kenwood Excelon KDC-X599

Sony MEX-N5100BT

Good luck!!