LOC, oem system, a.m. amp

i would like to connect my sub and amp to my OEM sound system.   the HU does not have pre amp outs, thus i could see ways to connect it

1LOC,  splice into the + and - of the rear left and right and go thru a LOC which goes to the amp then to the sub

2a high level input amp, splice the + and - of the rear left and right directly into the amp

my question is what is the difference in the 2 ways?   i know if i use method 1 that my rear speaker levels (and my front speakers too?) will drop but how much, and is it according to how hard the amp is driving?   if i use method 2 will i fry my OEM HU?  and in turn will that affect the dB level of all of my speakers? (front and rear)   does the LOC isolate the 'pull' the aftermarket amp has on the OEM amp?

a third option mentioned to me is to use a multi channel amp.  take the rear feeds into the amp and use ch1 and ch2 for the rears and ch 3 and 4 bridged for the sub, but then i am spending a lot more money and doing a lot more wiring.