sqrt [ (RMS of amp) * (load applied) ] = output voltage
so for you that's:
sqrt [ 800W RMS * 2ohms ] = 40V
and that should be measured on the output terminals that you're connecting your sub to.
BUT if you're amp is over-rated you could still clip //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif theoretically though, if your amp can do a clean 800W, you should be reading 40V to get that output.