Audio Systems: Subwoofers, dual voice coils, ohm stable

Question
I am looking at buying 2 Pioneer TS-W306 12" 4 ohm subwoofers and am trying to decide whether to go with single or dual voice coils.  I have a Clarion DPX1800 amp which provides 800 watts at both 1 and 2 ohms.  The manual says it is 1 ohm stable but 2 ohms is recommended???  Why is 2 ohms recommended if it is 1 ohm stable, and how can it provide the same wattage at different impedances?  Also, is there any loss in sound quality with 1 ohm when compared to 2 ohms?  Thanks for your time.

Answer
I think what they're trying to say is that the amp is really designed for a 2 ohm load but wont "blow up" if run at 1 ohm, get what I mean?  In other words, I'd head their warning and stick with 2 ohms.  Usually the formula for distortion vs ohms is that every time you cut the ohms in half, you usually get 4 times the distortion.  so if you run your amp at 2 ohms you'll get alot less distortion.  at 1 ohm you may get a watt or 2 but not worth it.