I don't think you'd have to upgrade the headunit. I totally understand wanting to keep the stock unit. I'd keep it stock too. These days (IMHO), OEM headunits aren't nearly as bad as they were 10-15-20 years ago. And with all the extra doo-dads like steering wheel controls, heads-up displays, etc, it can be exceedingly difficult to do so anyway.
So just so I'm clear, you're tapping into the rear channels, right?
The catch is how to get two sets of outputs from that. One for the sub amp, and the other for either the 6x9's directly with some kind of passive crossover, or with the rear 6x9's amped.
I'd go with your option 'B' ...
The LOC will take the two line-level outputs (L+R) and convert it to two RCA plugs (L+R), which is a lot easier to work with. Then I'd buy two RCA splitters, and split the outputs into two pairs (2L + 2R). I don't think you would have a problem splitting the signal like that. I seem to recall doing this a few times in various setups and never having any problems.
Connect one pair to the sub, and the other to an amp. Any little 2-channel amp would do so long as it has a variable high-pass crossover. I didn't pick the Blaupunkt one for any reason other than it was the first one that popped up on Crutchfield's site and it was pretty cheap. You could probably get one even cheaper at your local big-box store. Run the speaker-outputs from the amp to your rear 6x9's, no surprise there.
If you went with that Blaupunkt at $60, the total cost to you would probably be less than $100. A LOC and RCA cables are pretty cheap (RadioShack). You'd get fully independent crossovers to play with (sub & rear), and enjoy a nice power boost to your 6x9's. The only problem I can see here is that it might take a bit of work to balance out the sound volume between the front speakers (running off the HU), the sub, and the rear 6x9's. For that reason, if you use a rear-channel amp, make sure it has a level control on it (most do).
I would guess that you can probably power the little 2-channel from the existing power lines & remote turn-on that you have run for the sub amp. It depends on the gauge of wiring you used and how it's fused. But I can't think that a little 2-channel amp, especially one high-passed, would draw enough current to really make much of a difference.
