Resistance vs Wattage Question

A friend of mine just picked up a DVC Infinity Kappa Perfect DVQ sub and I was helpin him look for a sutable amp for it.  Well he can run it at 2 or 8 ohms and it takes 400W RMS.  Now all the 2 channel amps I have seen are rated at 4 ohms in their bridged mode. My question is, if you were to run this sub wired up at 8 ohms on an amp rated at 400x1 @ 4 ohms, will the power going to the sub be cut in half?  Looking at Ohm's law, it seems this is the case, but I was wondering if the practice verifies the theory.  I would really like to set him up a nice system and have him not worry about anything.  Thanks for any help on this....

Oh and would you guys reccomend used the line level inputs, or using a line output converter.  We're going to be hooking it up to a stock head unit and I just wanted to know some opinions on both sides.  Thanks again.

have you guys looked into a mono amp at all?

yes the wattage would be half

It dosn't really matter if you use a line level convertor or the amps high level inputs. If you buy a descent amp then the high level inputs on the amp should be fine. I would use a line level convetor if you dont have speakers close to your amp that are are accessible or if your amp dosn't have high level inputs. Nothing is wrong with using a line level convertor, it should not degrade the signal, unless your rca cables are run next to power cables and that could cause interference. If your amp has high level inputs then it bassically has a line level convertor built into it.

For the infinity sub I would use a good mono amp like the alpine MRD-M301

If your on a budget also look at the alpine mrp-m350, it puts out 350 wrms at 2 ohm, and the next step up is the mrd-m301 (this years model) and the mrd-m300 (last years).

They all have high and low level inputs.