yeap underpowering is a bad ide for your speakers. however, if you have a factory deck, take a digital multimeter and read the resistance on the speaker. I know factory speakers say 4, but its never true. they are always higher than wat they are rated for. If your speakers are 4 and tweeters are 4 ohms, wiring in paralle will give u 2 ohm load(too low for your deck) if you wire in series, you get 8 ohms(safe to use, underpowering even MORE than before)