What amplifier power does the X v.3 12 need?

It is rated at 2500 watts RMS and 5000 watts peak. Plan for a strong amplifier and upgraded electrical, since clean power near the RMS rating is what the sub is built for.

What size enclosure does it work in?

It is designed for small ported boxes and can also run in an enclosure up to 50% larger if you want more low-end extension. Port tuning matters more than box volume alone.

Should I get the dual 1 ohm or dual 2 ohm version?

Pick the coil configuration that gives your amplifier a safe final impedance. A dual 1 ohm sub wires to 0.5 or 2 ohms, and a dual 2 ohm sub wires to 1 or 4 ohms.

How is the v.3 different from the v.1?

The v.3 has a larger 220mm motor versus 190mm on the v.1, improved motor ventilation and the new Mega-Roll v.2 surround. Sundown says the result is a more linear and rugged design.

Is one order one subwoofer?

Yes, a quantity of one is one subwoofer. It does not include an enclosure, wiring or amplifier.

Is it good for sound quality or just SPL?

It is tuned for ultra-deep bass and high output, but the linear suspension and low distortion make it usable for daily listening too. It is not a lightweight, efficient sub for modest systems.