!5 watts of power, is a very small amount of power. On top of that you probably, have loudness contour on and the bass knob at 3:00. The distortion you hear is the amp clipping. I do not know how efficient the Advent 3's are, but if you want to use this unit you will need more efficient speakers. I do believe in using a quality powered sub, but cheap subs just don't cut off any signal LF going into the amp, which would help you with not loading the amp with LF. IMO anything less than 50 watts, is hard to get good bass or high levels, with a medium efficiency speaker.
EDIT: It is not a matter of taking the the LF away from going into the speaker, You have to keep it from entering the amp in the first place. LF is eating up the power of the Marantz.