[mikegett]

You don't splice all four wires togeather. I have said this before and I will say it again. BAD, BAD, BAD. The tweeter is on one channel of the amp (4ohm) and the woofer is on a seperate (2ohm)channel. This is called a bi-amped setup. The amp has a built in crossover dedicated for each channel. This means that the tweeters only see the highs and the woofers only see the lows. Your new speakers are currently setup with only one channel and thus are only getting one set of frequencies. You have two choices. Buy a set of bi-amped speakers, or remove the monsoon amp from the equation. The second is the easiest method. The HU is still sending out about fourteen watts to the rear channels. Trace the rear, left and right speaker wires to the monsoon amp. Cut these wires and send it directly to your new 6x9's. You won't have the 25 watts from the monsoon amp, but you will have all of your frequencies.
Warning: By splicing all four wires togeather, you will be running two amp channels to the same speaker. This will result in blown speakers, and could damage the amp. In fact, the added resistance could cause a electrical fire.