Need some help with transformers (impedance matching)

[cuztom15]

Ok, so i have this circuit:
12v DC to 100w Amp (split 50wX2) at 4ohms or 8ohms (that supposedly pulls 10A at full load) going to two 4"speakers rated for 300w Peak and 80w rms both at 8ohms. My whole plan is to either get the speakers down to 4ohms. Now after doing my calculations of 12^2/8=18w (although the manual says 13w) and 18w/12v=1.5A, I figured that I would need a 1:1.5 turn ratio to produce about 50watts of power to effectively increase the volume of my speakers.

The problem im having is finding the right transformer that is either a 1:1 transformer with an 8ohm primary tap and 4ohm secondary tap that will handle the circuits specifications or a 1:1.5 transformer with 8ohm primary and secondary taps that will handle the circuit.

I have been to jensen, mouser, and newark, and I cant seem to find anything using their filters, its a tad frustrating. Any help would be greatly appreciated.

Mock circuit pic for reference....