Welcome to the forum and enjoy your 2012 JGC. But, most of all congratulations on your forth coming hitching.
Let me be quite frank with the JBL MS-8. This weekend I have experienced problems with it again. This is my 2nd unit from HK. The first one, had a problem that produced a processor error that did not let me finish the initial setup. Now recently, I am loosing channels. I did a complete reinstall and now I missing my rear D pillar speakers. I will speak with tech support on Monday for any insight to fix it on my end or it has to go back for a repair or a 3rd unit. HK is not the same company that it used to be. Plan B, I am looking seriously into the Alpine PXA-800 again.
Now on to your MS-8 questions:
The best way for a SQ is to use an aftermarket HU and directly into the MS-8. However, coming out of the factory amp into the MS-8 LOC will give you a very good improvement. Just tap the sub, front door 6x9 and tweeter. Just remember, the amp in the MS-8 is not designed to push a lot power, therefore, make sure you get speakers that have a high efficiency ratings.
With regards to your future plans, very nice selection. The PDX-V9 5 channel is a nice amp. The negative feedback on it comes from not hitting fast enough on some midrange frequencies for certain speakers. The JL Audio has the edge in that area. Look into the new XD 1000/5 v2.
Since, you do not have a surround system in your JGC, I would consider looking at other DSP's. throwing in another 2 channel amp to drive the rear door is not that much and it will be a better sound quality. In addition, you can setup the rear door in a band-pass xover filter and have excellent control in tuning the rear.
Hope this helps.
Best for now.
Laredo X, Trail Rated, QTii, Sele-Terrian, Quadra Lift, Off-Road Adventure Package ii, Nokian Rotiva AT, Tow, Custom Sound System ( HAT speakers, JLAudio, Pioneer, JBL MS-8 & RF 3.sixty.3).