Re: First build! 2008 Scion xB "SQ" build.
Nothing wrong with JBL's products but I do think you could do better for about the same money for the whole system.
For an SQ build, I would consider nixing the rear fill completely or, at least, leaving the factory speakers in place and running them from the head unit with a simple passive crossover if, after the system is built you feel a need for rear fill. But regardless, the best SQ you'll get is from a strong front stage that's at (as close as possible) ear level. And if you're concerned about rear seat passengers, a strong front stage is still enough to make rear seat listening enjoyable.
That said, if it were me I'd still be planning on a 4 channel amp but I would run front channel components active from it. If your 4 gauge is ofc then you're okay with keeping it provided the power you're running doesn't go up from your initial list. But I would do a big 3 and put in a second battery, although how large a battery needs to be decided once you've set the amp selection in stone.
For the sub stage, first thing I have to say is that the JBL sub is more form than function. It's a sick looking sub but sick looking doesn't indicate good performance and because JBL isn't forthcoming with TSPs on it, I can't help but think it's a bit of an underperformer. I have experience with the GTO line of subs and for entry level subs, I would recommend them. But this isn't an entry level sub so, I would shy away from it. Matter of fact, I'd keep the Qoundqubed sub before I'd buy that one. I might change my mind about that if I could see some real specs on the GTI but again, JBL doesn't publish them for a reason.
For amps and speakers, I would look to another company and since I'm a CT Sounds dealer, obviously I'd steer you to them or Execution Audio, which is their entry level line. But my reasoning isn't purely self interest and I'll explain....
Go to a few car audio competitions and see what people are running in the lanes. Or, got on youtube and look at videos from competitions. You won't see much mainstream gear there and you'll see even less of it with trophy's standing beside the cars. In simple terms, if you want professional performance, run what the pros run and if you want mediocre performance, run what's popular.
And once you've done some research you'll be surprised at how inexpensive a lot of professional gear is compared to mainstream gear. Often times it's cheaper for more power/performance.
If you're interested pricing on CT Sounds, I'll check for a local dealer for you and if there is none, I'll be happy to help you out. :-)
Owner, Audio Anarchy, LLC
2915 Youree Drive, Shreveport, LA