the JBL BP series amps are rated to be 1 ohm stable, but the output is regulated so that tehy put out the same power as with a 2 ohm load. ideally stick to a 2 ohm load at the lowest for nominal impedance.
you can get a single 2 ohm sub (or dual 4 ohm coil) rated for 1200 watts, or get a single 4 ohm sub and run the amp at 4 ohms, and 600 watts which will give you a much cooler running amp. you could also look at a pair of 4 ohm subs so that each sub gets 600 watts. a pair of 12" or 15" Adire Brahmas would be a good match.
you could maybe try to get 2 12L5 dual 4 ohm and run the amp at 1ohm giving 600 watts to each sub which is exactly what they need, i dunno just a suggestion