Bass in music rarely sees 0db, so most people use -3 or -6db to set gains. I use 0db because I like the reference point it gives me due to the fact that all my music is bass boosted through Audacity.
Food for thought, if you set gains with 0db tone, and the song you play doesn't exceed -6db, you're only getting quarter power. So 1000watts is actually 250watts. Therefore, you have to turn up the volume on the HU some more to get closer to 1000watts.
If you set gains with a -3db tone for 1000watts, now that -6db tone in that song is giving you 500watts.