Oh, I messed up...
You could use the Pioneer amp just on the front speakers... It should power them pretty good.
I was thinking the specs were MAX ratings
So if you did hook the Pioneer up to all 4 speakers (2 on 1 channel), it would be in 2ohm mode pushing 120W to them... I'll assume that'll split up and you'll have 60W RMS to each speaker... but you will lose the fader function of the head unit.
-OR-
You could risk it and hook the American Legacy up to all 4 door speakers pushing 50W RMS to each one (but that could be overrated), and hook the Pioneer up to a sub.
There's a lot of subs that still sound good with low power. My old-ass RF 15 is one IMO... it's an 8ohm SVC sub, so it halves the power of my MTX amp to about 150W RMS. The sub's RMS is 150W also, so probly why it sounds good.
In bridged mode, the Pioneer is rated at 240W RMSx1 at 4ohms (2ohm not listed, so probly not stable).
So you can do the following:
Get 1 4ohm sub with an RMS rating of ~250W+
Get 1 8ohm sub with an RMS rating of ~125W+
Get 2 4ohm subs (~125W+ RMS each) and hook in parallel [to get 4ohms across both]
Get 2 2ohm subs (~250W+ RMS each) and hook in series [to get 4ohms across both]
I kinda confuzed myself on the 2 last setups, let me know if I'm wrong!
Anyway, you can do stuff like that with the Pioneer.
But remember my friend's American Legacy amp (looked just like that one, with the 2 windows) caught on fire... not good if that happend under your seat!