in the wifes 6G, I just put a RF 300.1 to an RE REX12 in a 1.5@35 box. stock HU, stock speakers, faded it to the front a bit so you cant tell anything is coming from the rear and I think it is more than enough. also if you dont so what most peole do and crank the gain so damn high that your subs are already loud as hell when your volume is 1/16th the way up, and set it so it actually increases in volume with the system all the way to however loud you listen to it, then it doesnt overpower the speakers. when the volume is below 1/4 and you are trying to listen quietly, you barely tell the sub is there but turn it off and then its obvious you are missing something. around 1/2 the sub is pounding pretty good but not overpowering anything and when you really wanna crank it, around 3/4 volume and the sub is maxed out (well, its where I set it to be) and the speakers are really loud as well so it still sounds fine.
but if you are trying to isntall a really nice system and are going to go crazy with subs/amps, then why not amp some comps up front or something. but for the average person who just wants a little bass. there is no need IMO to do anything else than a sub/amp