Oh, you have the amp already? We're trying to help, but we need information. What we're given seems to be confilcting.

400 watts RMS? A converter should be able to do it for one of these amps. 400/120 = 3.33 amps. At full power it might draw more like 5 and put out 3.33 due to the inefficiencies inherent in the conversion. I don't remember off the top of my head who makes better models as I did more research in the other direction.

Quick look , 540 watts @ 4 ohms for $250? No conversion, so no worry about blips in the power supplied. BTW, what sub(s) will this be for? That'll help. I'm curious as to what amp this one is too.