The sound change from the airbox is the increased sound of air entering the engine. Sounds good but won't get you where you want to go.

The Y pipe doesn't do much, X pipe will change the sound some but it is unlikely there will be popping or crackling.

If you are talking about the popping or crackling when you let off the gas or between gears that is often fuel vapor that ignites in the exhaust system. The closer to the engine the more likely there will be popping or crackling. I suggest you drop the exhaust post cat and see if you get what you want when you rev the engine. If you do then go to electric cutouts that you can adjust how much they are open as has been suggested. I get popping and crackling from mine and when they are closed I still have the no drone Dinan exhaust that sounds great to me on it's own.

I have to disagree. I just did this in my X5 with neglible results....in fact I would call them disappointing results.

^Did you keep your factory mufflers?

Yes

Next step would be to get some 4.8iS mufflers then - I'm happy with factory resonator and 4.8iS axle back exhaust for the throatier mufflers - some people like both, but an Xpipe is a cheap way to see if you get the sound you're looking for

Exhaust sound is subjective so it differs from one person to another. I am only commenting in what I have heard after the X-pipe has been installed. In my opinion it sounded pretty good with the loss of center resonator. I would start here first before eliminating rear mufflers. If that is not enough then loose the rear boxes.

+1 on your set up