From what I've read in the car audio forums, a good pair of front speakers powered properly is ver ideal.
If that is true, I would tend to gravitate towards a nice 2 channel power amp with a sub output. Or something along the lines of a 4 channel amp that with rear channels that can be bridged to power a sub.
Basically, it is going to be a project that my son and I will be doing.
No big boom, just nice and refined detail. My kid is not into the hard bass line type of thing, he is used to my home audio. He grew up listening to NAD and Sonus Faber gear, if that is any indication of the type of sound he is looking for and me as well.
We do not want to use a sub box that will be taking up space. We will be using the stock sub location which is 10".
Right now the HU is a Pioneer 6900UB and the speakers are Infinity Kappa 62.7i and the sub is some sort of JL Audio 10 inch. Haven't listened to these so I don't know. So we are looking for a decent amp.
As far as budget,no real worries, but nothing too extravagant. Something that will adequately power the set-up without worrying about distortion.
I'm kinda new to the car audio thing so I am starting on my son's SUV. But I am familiar with installations and car electrical power systems.