get a cheap dmm if you don't have one. download the 0dB 50hz test tone from realmofexcursion.com burn it to disk.
Using the formula V=Sqrt(PxR), and assuming the 1505 is a little over-rated (better safe than sorry)... V=sqrt(800x2) = Sqrt1600 = 40V
Now - disconnect your sub, and disable any other speakers if it's convenient b/c you're going to be playing that tone at full volume. Put the cd in on repeat. Turn your HU volume up to the highest point you ever listen at. The idea is to get maximum output from your pre-amps w/o clipping them. Measure voltage at the amp - adjust the gain until you get 40-41v.