I initially let my receiver set the level using it's auto setup. It set the sub at -4db so I guess the sub's gain was too high. I tweaked the LFE trim in the receiver to 0db, so relative to the receiver it is now 4db hot. I can adjust the level on the fly via the remote in case the bass level seems too low/high.

If it sounds good to you, set it where you like it!