stereo wire w/copper on both wires; is wire w/lettering + or - ?

I assume you are refering to Stereo Speaker Cable, W is only a marking printed on the cable to indicate this side of the copper core from start to end. If you choose to use it as + , fine, go ahead, the other end will be + (positive or red terminal )also, so that you won't make a mistake on the other end . Then the other core of the cable without the W marking will have to be - (negative or the black terminal)

The marking is to help you keep track of both ends of the wire. If YOU decide to hook up the wire with lettering to the "+" side, then the wire with the lettering becomes the "+" wire and you then can hook up the other end properly.

As a rule of thumb in my house, I always use the marked wire for the GROUND or "-" terminals. If you get red and black wires, then black is ALWAYS considered GROUND or "-".

You need a plus wire and a minus wire for each channel.

You can make either one the plus or the minus.

USUALLY LETTERING OR RIDGES IS POSITIVE ... BUT REALLY MAKES NO DIFFERENCE ... AS LONG AS YOU ARE SURE TO CONNECT THE SAME ENDS OF SPLIT WIRES TO APPROPRIATE SPEAKER OR AMP TERMINALS