I'm curious why JL would void a warranty for only using one coil? I don't doubt that their literature states something like that but why? While it is not common practice, using one coil is an perfectly acceptable method, as long as you realize that you are halving the power handling and some other parameters.
If I were you, I would either get the JL10W3 (that is if you want brand name subs, if name recognition is not important as sound quality let me know) which i believe comes in more voice coil arrangements.
Actually, if you decide to get the 10W6, attach them so the amp sees the 4 ohm load. You will have 250watts to three subs. This should be more than enough, unless you are trying to make interior parts fall off from the vibration. You would probably get away with two W6s with all VC wired in parallel for a 1.5 ohm, but as mentioned earlier, anytime you dip below the amps ratings you are looking for trouble. Plus, you would not hear the difference between 2 subs with 250 watts to each, or 3 subs with 250 watts total.
