Ohms is NOT how much power is getting to the amp. The amp power draw is measured in amps. Ohms is the impedence measure, which tells how much resistance there is in the flow of power from the amp to the speaker/sub.

That thing about efficiency is also false. Different amps have different power outputs at different impedences. Just because you lower the impedence doesn't necessarily equate to more power. (Example: My Xtant X1001 provides max power, 1000W, at both 1 ohm and 2 ohms configurations.) Plus, not all amps are designed to handle a 1 ohm load.

Cambridge, to determine the amp that best suits your needs, you'll need to figure out what voice coil configuration your sub is (dual 2 ohm or dual 4 ohm). If it's dual 2 ohm, you can wire it at 1 ohm parallel or 4 ohms in series. If it's dual 4 ohm, you can wire it at 2 ohms parallel or 8 ohms in series. Also, keep in mind that the 400W RMS rating for the CompVR is for the recommended sealed enclosure. If you're going to get a prefab bandpass box not built to spec, it would affect the power handling capability of the sub, so go easy on the gains.

Here's a good amp if you have a dual 2 ohm sub...Hifonics Zeus ZX4000 for only $141 shipped.
Or check out the Kicker KX450.2. It's also under $200.

Here's one if you have a dual 4 ohm sub...MTX Thunder 421D.