What you are asking for is on the topic of "target" or "house" curve.. That's where your personal taste for tonality will come into play a bit. There are some somewhat established norms, but in the end, it's what sounds balanced, smooth and coherent to you. I've been told all kinds of stuff like "flat from 200hz up" and 20db total delta between 20hz and 20khz. I think after enough tuning, you'll establish a tonality that's balanced for your car. I don't believe there's any single curve that's right for all cars or systems. Crossover settings are relevant in that crossover regions you want to keep an eye out for any destructive FR stuff that may happen due to phase or timing being out etc. Other than that, the overall tonality should be smooth without spikes or peaks (narrow unavoidable dips can be forgiven). And possibly even more important, left=right.
I had measured my drivers individually without any crossover set. As it turns out, my sub shows a huge dip in FR at 90 Hz. Has anyone an explanation for this? It is a sealed footwell sub, 8" driver in a small enclosure (approx. 10 Liters). Before any measurements I had set the crossover to 99Hz because it sounded best to my ears. Now, seeing this dip, I have my doubts if 99Hz is really recommended. I should probably cross it at 78Hz to avoid that dip!? What do you guys think?