I couldn't find any info for a Rockford T1000bd amp, but I found info for T10001bd. Is that your amp? If so, to get the most power out of it, you need to present it with a 1-ohm load.
I know nothing about Fi BL subwoofers. Are you saying you have the option of either getting a 1-ohm dual voice coil model or a 2-ohm dual voice coil?
If that is the case, we also need to know how many subwoofers you plan on getting. If just 1, then you will want the 2-ohm version, and wire the voice coils in parallel, to get a 1-ohm load.
If you are getting 2 subs, then you will want to get the 1-ohm version, and wire each voice coil in series on each sub, so each sub is 2-ohm, then wire each sub in parallel with each other, to present a 1-ohm load to the amp.
