Sync problem is being caused by your connections. You have created two different signal paths for the 3 speakers.
Oppo HDMI output > Marantz > Center Speaker
Oppo Optical output > DAC > Preamp > Amp > L&R Speaker
The issue is not using different amps, but using 2 different outputs on the Oppo. Both amps must receive a signal from a single output.
Oppo HDMI output > Marantz > Center Speaker
Oppo Optical output > DAC > Preamp > Amp > L&R Speaker
The issue is not using different amps, but using 2 different outputs on the Oppo. Both amps must receive a signal from a single output.