What subwoofer should I pair with the ACS-1000D?

Match a single sub whose RMS rating and impedance fit the amp's 1-ohm output. A dual-voice-coil sub wired to 1 ohm is the natural fit. Check the sub's power handling against the amp's RMS rating at 1 ohm.

Can I run two subwoofers on it?

Only if they are wired together to present a 1-ohm load the amp can handle. It is a single-channel amp and not bridgeable, so it cannot run two separate loads.

Does it have a built-in crossover?

The amp's response tops out at 250 Hz, so it is intended for bass only. Set the low-pass filter to match your sub and cabin, and keep full-range speakers on a separate amp.

Can I use it in a boat?

No. It is not rated for marine use, so moisture and vibration in a boat are a risk. Choose a marine-rated amp for boat installs.

Is Class-D good for car bass?

Yes. Class-D amps are efficient and run cooler than Class-AB, which suits the long, heavy bass draws of a sub stage. Some listeners find Class-D less warm, but for subwoofers the difference is small.