analog would mean that the deck produces an AC wave that plays directly to the speakers. That does not necessarily mean the deck does not have sensing for dead speakers (when you would need resistors) some decks send a digital signal to a amplifier that decodes the data, processes it, amplifies it, then sends an analog signal to the speakers. This can be done either over copper wire, or plastic fiber optic cable depending on the design of the car. GM does not use fiber optic cable, so you are in the clear there.
i have not done a 18, but i have done a 17 and resistors were not required, at least when replacing the speakers with an aftermarket with a 4 ohm voice coil.
I have used resistors on that truck before, but only when amplifying the door speakers. The resistor was used to keep the factory tweeters from blowing your eardrums with the door chime.
while i have not personally used the summing device you are talking about, i do know about it, and have heard lots of good things about it. I will tell you that the factory radio on those new trucks really cuts the bass hard, without some kind of signal processor, your final results (sub wise) may be underwhelming.
Right now the plan looks something like this
Line Out Converter =
Audio Control LC2i
Front Door and Dash Speakers/Amplifier =
1x Pioneer GM-D8604 Class FD 4-Channel 600W amp
1x Pioneer TS-D1720C 6.75-Inch D-Series Components
Rear Door Speakers =
Under Rear Seat Subwoofer/Amplifier =
1x Pioneer GM-D8601 Class D Mono 800W amp
1x Sundown Audio SA-10 D2 Rev 2 (10" Dual 2-Ohm SA Series Sub)
Possibly a LC7i over the LC2i, use the component tweets in the dash position, 6.75" in the front doors, maybe sending power to stock rear door speakers