Please tell me how I can check the impedance of a speaker with a basic multi-meter.

I asked the guys at BSW if they knew and they confirmed that the factory DSP speakers are 2ohm and the subwoofer is 8ohm. I'm assuming that they are individual 8ohm speakers.

Would a replacement sub be better? You're probably right. Have you seen what they charge for their boxes? I'm going to make due with the speakers I have for the time being.

Total invested:
Kenwood HU: $259
Kenwood 5-chan amp: $288
3-way crossover: $50
HU install kit: $20
Wire and connectors: $10
Total $627

Added Bluetooth connectivity for Android device, USB input, 1/8 jack, retained factory appearance. Happy wife...