multimeter

I am having an issue, I was able to set my subwoofer amp gains to the calculated 50v fine (nvx xad13) (sounds and works great), I go to set the gain on my sundown sdx 200.2 and gain maxed at the same volume I set my sub amp (same volume on the same headunit I had my last setup running on fine)...

Can you use a multimeter to determine polarity on wire that is unlabeled (potentially while music on)? Wires were connected to factory rears to run to amp high level in. Those bare wires will power a test 5x7, just unsure of polarity when wiring my high level adapter.

I have a Pioneer GM-D8601 amp which is rated at 500w x1 at 2ohms. I have a Kicker 40CWRT101 10 inch sub with dual 1 ohm voice coils with 600 watt max RMS. I have the speaker wired in series for a 2 ohm load (double checked with multimeter). So I do the math figure that I should set the gain to...

I was setting the gains on my amp a boston gta-704 rated power is 70x4 watt rms and I was setting it to 65 watt rms at 4 ohms the math says the multimeter reading should be 16 or so I rounded down but to get this reading I had to turn the gain up to around 3/4 and on just about every amp I have...

Just looking for a cheap, and for the most part accurate meter. Also, recommendations for meter's circa 30 dollars? http://www.ebay.com/itm/Electric-Multimeter-UT203-UT-203Meter-DC-AC-Digital-Tester-Clamp-Electrical-/271220733393?pt=LH_DefaultDomain_0&hash=item3f2603f5d1