Are you Zu's bottom ported? are you on wood floors or carpet?
when i moved my Superflys to a carpeted room i noticed the bass totally went away because the bottom of the speaker needs to be an inch or so off the ground. (the zu site has the recommended distance) i ordered some Herbie speakerfeet to raise my speakers off the ground and BOOM bass came back. had nothing to do with my amps that i kept switching. hope that helps.
http://herbiesaudiolab.net/spkrfeet.htm
when i moved my Superflys to a carpeted room i noticed the bass totally went away because the bottom of the speaker needs to be an inch or so off the ground. (the zu site has the recommended distance) i ordered some Herbie speakerfeet to raise my speakers off the ground and BOOM bass came back. had nothing to do with my amps that i kept switching. hope that helps.
http://herbiesaudiolab.net/spkrfeet.htm