Have you installed a dummy jack into the send? Also you need to turn on the FX loop relay using the switch on the back panel. Running direct though the Return, you would have to use the master volume control since the channel settings will have no effect on the output when they are bypassed. Not sure what the output level on the preamp you are using operating at. If it is instrument level, it will sound weak. The FX loop needs line level signals if you want to have the same amplitude of signal that is supplied by the Send jack.