I agree with the 4 ohm 8ohm problem, but I have always been skeptical about 400/500/ even 1000 watt ratings. What is really important is the RMS of the speaker and the sensitivity. The 90bd sensitivity speaker needs almost twice the grunt to reach the same volume level of a speaker sensitivity rated at 92bd. If you have miss matched sensitivity the system defaults to the lower sensitivity. You can also get clipping, which is seen on an oscilloscope sine wave where the tops of the wave have been clipped off the tops and the wave has a flat top. There is also the problem of speaker wire. They should all be the same gauge. Best is all the wire off the same roll. Although there is a reasonable (good science) argument for having larger wire smaller gauge number for longer runs in the same system. You are not going to get audiophile sound from a car, and never in a convertible. So what you should be looking at is standardization: all the same brand of speaker, the same gauge wire, good quality plugs, preferably soldered or compression fit. The same sensitivity for all speakers. But then once you start using your streaming unit be it phone, or what ever that has used a compression algorithm, and not a wave file, then there will always be a compromise. So look at your RMS for your speakers, not peak. Peak is when they blow, RMS is the max before the sound starts to degrade. Then get the sensitivity of the speakers all the same. And last, a 10 inch under the seat active sub. That means it has it own amp so the sensitivity is not as critical.