Hah, you can go ahead and PM me where you found the PDX-5 at a great deal, the lowest I can find is ~370 New on eBay, and even then you don't get an alpine warranty. Might as well save another 80-100 and buy used.
My ZX350.4 was a nice amp while I had it- no major complaints. It had enough power, but it seemed a little harsh. I was/will-be using Pioneer D series 6.5" components front and polk DB691 (or something to that effect) 6x9 for rear fill with no sub. I also couldn't get a decent blend, and I attributed a little of that to the 350.4. Mids lacked up front, bass was somewhat flat. I've heard each set of speakers sound better in other cars than they did in my car. There are so many variables though that its hard to pinpoint a real culprit. My biggest complaint was that it didn't fit under my passenger seat! Although that wouldn't be a problem in my new corolla.
I suggested the order Battery-Inline fuse-4ga->fused distro-dual 8ga->amps because you can often get a distro that can take 4ga or 8ga output. Then, if you decided to upgrade to a more powerful sub amp or full range amp, you only need to switch out a small piece of wire between the amp and distro, and up the fuse. As it is, you've paid for 2 separate inline fuse holders, one or both of which may need replacing in upgrade, and decent ones are generally 20$ or more.It will of course work fine using 2 separate lines/fuse holders as they have wired it, and you will have no noticeable difference in power or any other term of measurement by changing to a distro block.
The only possible difference as it exists today would be if the wires look cleaner run as you/I suggested.
The biggest nugget of info I got out of that site is the importance of fusing where wire gauge gets smaller. For instance you have 4ga going to distro, if you output from a non fused distro with 8ga wire, that wire is protected at too high an amp rating, meaning it would go up in smoke long before that initial fuse was tripped. Having said that, a 4ga wire would still be properly protected. Its about protecting the wire, not protecting the electronics at the end. They have their own fuses
As for dual amps for dual setups, it likely follows home stereo equipment protocol: Its better to have things that do individual tasks than an all-in-one. You used to have a tuner, amp, preamp, now you have a receiver. It didn't necessarily go that way for SQ, its more about space, simplicity, and affordability for people. Look at the popularity of HTIB systems. For me and the PDX-5 its about space. Stealthy, under the seat, powers everything and does it with class. If you use mono amp output you have to wire in series or parallel accordingly to achieve the proper resistance for the amp. If you've wired properly, I think things would still go according to plan even with a 5 channel. That is, after all, what you are doing with the 750.1. The kicker will be affording you with more power than the output of the PDX tho, possibly resulting in more headroom, and higher attainable volume levels with less distortion, if the amp can hack it.
Sorry for the rambling post.... So your battery is actually in the trunk? Convenient!