where:
Rt= total resistance
R1= first resistance
R2 = second resistance
so if you have 2 4 ohm speakers in paralell you would
1\(1\4+1\4)
= 1\(1\2)
which = 2 ohms
i find callifo's method easy if you only have a few easy resistances to work with otherwise i use the method i suggested.
funilly enough i know people who can only really do it one way or the other no matter what the problem...yet i use whichever method "seems" easiest at the time for me...
anyways i'm just talking crap
as to what amp
those jaycar responce amps are pretty cool from what i've heard about them, good bang for buck or so people say.
as to the sub and 6x9s, from what i've been told its not a very smart idea as they sound crappy together(as squid was saying)