For what it's worth on page 10 of the XLS-5000 manual it says use 16awg wire for lengths up to 25 foot.
2.5 Choose Output Wire and Connectors
Crown recommends using pre-built or professionally
wired, high-quality, two-conductor, heavy gauge
speaker wire and connectors. You may use 2-pole or 4-
pole Speakon® connectors (Figure 2.5 ), banana plugs,
or bare wire for your output connectors (Figure 2.6). To
prevent the possibility of short-circuits, wrap or otherwise
insulate exposed loudspeaker cable connectors.
Using the guidelines below, select the appropriate size
of wire based on the distance from amplifier to speaker.
CAUTION: Never use shielded cable for output
Distance Wire Size
up to 25 ft. 16 AWG
25-40 ft. 14 AWG
41-60 ft. 12 AWG
61-100 ft. 10 AWG
101-150 ft. 8 AWG
151-250 ft. 6 AWG
The speakon connectors inside the amp are connected with red and black 14AWG wire.
In the perfect scenario you provided you might find out that you are not really pushing the amp constantly to 2500W, or 70+ Volts for each channel. Basically, find some test tones, hook up your meter in ac volt function, and measure the amp output at your listening level. Keep in mind by using a test tone does not simulate real use. I would just stick with the 12awg...
I plan on doubling up once my second roll of identical 12 gauge shows up. Unfortunately I found out my cable ended up being Copper-clad aluminium 12 gauge wire. My research shows that CCA has more resistance making it equivalent of 14 gauge copper. Luckily I got a killer deal on it otherwise I'd be bummed.
I wonder would it be better to use a separate 12 gauge run for each sub box or running a single run (but doubled up in parallel) then hooping the second nearby sub? The amp ends up seeing 2-ohm per channel either way. The longest run would be 55ft. I'm guessing the 55ft run would benefit from doubling up due to length seeing how I'm using CCA.
What if the first sub has a much shorter run than the second sub when sharing the same terminal jack? Can anyone think of any technical benefits/disadvantages?