Is my wire rated at a high enough current?
I am by no means an audiophile/speaker designer, but with the numbers that you gave, I would say no. 9.5 amps @ 600 Watts through 4 ohms resistive equals 2.65 Ohms reactive. and 19.5 Amps @ 1800 watts through 4 Ohms resistive equals 0.73 Ohms reactive. The amp numbers at 3 Ohms instead of 4 would be 10.3 Amps @ 600 watts stereo and 21.97 Amps @ 1800 watts mono. If I am wrong on that, someone please let me know. I am not too familiar with speaker design, and it has been a year and a half since I took an AC course. 
In general, less resistance equals higher Amperage. In my opinion, the 12 Amp rating would be cutting it a little close for the 9.5 Amp stereo design. This might be common practice and generally accepted amongst speaker system designers, but I wouldn't know.