Specs like frequency response, distortion, etc. are pretty useless for a vehicle installation because let's face it, our Jeeps are not exactly acoustic Gardens of Eden.
The only specs I pay attention to are the features included with the radio. USB, ability to control an Ipod, auxilliary input port, how many output channels, how many pre-amp output channels, center-sub preamp output channel, low and high-pass filters, etc. When I find one with things I want like a USB port, Ipod control abilities, and it has a user interface I can understand, that's good enough for me.
But since you want to stick with your current speakers, just make sure the unit is one of the higher wattage rated radios. The wattage rating can be all over the board so stick with a higher wattage output rating and you'll be fine. It's important you understand what kind of watts they are talking about too. RMS is the accepted way to rate an amplifier's wattage rating... like 22 watts RMS which will be louder than 30 watts Peak power. RMS is how much continuous power it can put out, Peak power is normally about double what its true RMS wattage rating is. So compare apples-to-apples when talking output power in watts.