So I've always replaced head units in all my old beater cars,  added amps,  speakers,  etc no problem with any of that.

I have a 16 tundra with the entune audio plus, (non jbl) and i don't want to do what I've always done,  which is butcher harnesses to get things installed
I want to keep the stock head unit so frankensteining something isn't an option.

I'm gonna replace the speakers and add a small 50 wattx4 amp for the door speakers, which is np,  but i also wanted to add a small underseat powered sub,  something like this:
http://www.crutchfield.com/p_777B8PTD/Sound-Ordnance-B-8PTD.html?tp=114&awkw=75735772345&awat=pla&awnw=g&awcr=48387319825&awdv=t

The amp and sub will both have to speaker line inputs as the head unit doesn't have preamps, which complicates things.
Ive never used a speaker line sub, im assuming i just split the left and right rear channels prior to them going into the 4 ch amp, and run one set to the sub and the other set to the amp and then on to therear speakers?

Will doing that cause the rear speakers to be much lower and quieter than the fronts?
The sub will be affected by the fader correct?
Will the sub be affected as well? Like even if you turn the gain up higher it will still be lower than it would have been without the speaker signal being split?

Or will it all be fine,  and no balance issues between front and rear door speakers?

Ooorrrr is there a better way of doing things that I'm over looking?

Dang, nobody knows??