I have the exact same setup. What I do is switch the duets input to take a line level signal (+4), and run the 710 through that channel. The 710 has dual gain stages. If you want a clean recording, keep the gain level low, and then use the output level control to increase the signal level. Always try and give yourself 12 or more (probably a little more) db's of headroom when tracking. The blend knob can have a big impact on the tone as well. If you want really clean and "sparkly" recordings, you'll want the blend knob set more towards the transistor side. Need warmth and some harmonic grit? Turn the knob towards the tube side.
What source are you trying to record? And what about your recordings don't you like? I know that I was getting way too much in the way of early reflections in my audio, so I did a little room treatment with some blankets. Seemed to fix my problem.