When using the 5153 or the SYN5050 the signal path is the same for me:
Guitar > RM4 or SYN2 > 5153 or SYN5050 > 2x12 spk cab
The master volume knobs on the 5153 are for each channel, they don't affect anything that you plug into the FX return, so basically the power section of the 5153 is...