You could buy a sound meter to get an idea of what the dB reading was for "Volume 5" or what ever you receiver displays. Adjusting treble and bass is entirely up to you. I set mine and leave it for everything, tv, movies, etc. The more important settings are the x-over settings, the ohm setting (6 or 8), and balance, imo. Some people like to mess with the equalizer settings as well but that can really be variable depending on the type of audio you are listening too and how it was recorded. I just keep mine at the default setting (what ever that was).
Some receivers offer an automatic calibration feature (Audyssey) that is useful, but I think it's real value is for higher end systems with separates and not necessarily for HTiB's. Some will disagree but with a little time, knowledge, and possibly a sound meter you can calibrate your system better I think than the automatic calibration.