[JackG]

Out of phase speakers definitely DO matter. If they are out of phase, one speaker cone is moving in when another is moving out. That causes a diffused sound image, and the low end (bass) is canceled out.

If you can see or feel the speaker cone, you can momentarily touch a 9 volt battery to the speaker terminals or wires. Notice which wire or terminal makes the cone move out when touched to the positive battery lead, and you've figured out which wire or speaker terminal is positive on your speakers and aftermarket wires.

Now if you can make sure that all the speakers are wired so that the positive amplifier out is hooked to the positive speaker terminal, they'll all be in phase, and will sound much better.

If they are all positive to positive, or all are positive to negative, you're OK. If some are positive to positive, and one, or some are positive to negative (out of phase), that's bad.