They were incorrect. There are 3 settings for the ECO mode.
Auto - Turns on automatically according to the volume position (on my X4400H it turns on at around 42)
On - Stays on full time.
Off - Stays off full time.
Below is a response from D&M on this topic. It was in an AVS Forum post I linked in another thread some time back.
"When ECO Mode is used, voltage for the amp stage decreases about 1/3 and amplifier stage energy loss decreases about 1/6 compared to ECO Mode being OFF.
It is recommend when using external amplification to leave ECO Mode set to ON. This will reduce power consumption to less than 70W.
When using ECO Mode ON at lower volume levels (under 45 Absolute) power consumption is about 1/2 as compared to leaving ECO Mode OFF. This is useful if using the AVR at night and you do not want to disturb others.
When using ECO Mode Auto, the circuit will control the power supply voltage by a relay. If the volume level is more than 45 Absolute the circuit will cut off. If the volume is less than 45 Absolute the circuit will switch on automatically. If listening at the threshold level (45) you will hear the relay clicking on and off frequently. Voltage for the amp stage does not change here, so power consumption would be the same as normal.
Thank You,
D+M Custom Install "
Did you have ECO mode on, or auto? As I mentioned before, if ECO mode was on, at volume -7 you would probably hit the limit set by the ECO mode and that could have been the cause of the distortion due to clipping.
If ECO mode was off and you are getting distortion then there is something else wrong, could be a defective unit though highly unlikely, your source material, media player, or speakers. In that case adding an amp won't solve it.