I've been reasonably content with the factory sound in my 2000 Xterra. That Camry has spoiled me, though. Its just the factory system, but the sound is really good. I didn't really even consider the sound in the Mustang. I test drove something like 16 cars (OK, four of them were different Mustangs) and most of them were totally boring. The Mustang was a late and very different consideration than the others, but I was hooked. So no regrets (other than I'd have liked the Candy Apple Red rather than Silver.)
But listening to the Shaker is just dissatisfying. I even parked it and Camry side by side and listed to the same songs. Its just not the same. I don't know enough to pinpoint what's different, but the sound in the Camry is much better. And I'd like better sound in the Mustang, even as much as I like the sound of the Mustang itself.
I do have a (second) inquiry into Crutchfield. Their first recommendation for a head unit, the Kenwood Excelon KDC-X496 gets bad remarks for complicated controls. I hate to pester them too much. I might buy the head unit from them but almost everything else is available elsewhere for 60% of their prices or less.
I think between you guys and a buddy here I will get a new head unit. The same buddy just offered me a set of JL Audio gear. A e4300 amp, a 1000/1v2 amp and two 10W6v2-4 subs for $1000. They're about 10 years old (one of the boxes has a 2002 date), but they were only used for a year and have been in storage since then in the original boxes. Seems overkill and more than I wanted to spend, though.