i tried but the sound is still flat
Three little "experiments"
1) Maybe use the variable preamp mode on su9 and turn it down to zero. Then turn the da9 volume all the way up. Finally start turning up the volume on su9 until the desired level.
2) Can also try the opposite. Da9 to zero, preamp mode on su9 to 99, start turning up da9.
3) Fixed preamp mode on su9, adjust volume on da9.
Which speakers have you got btw?