First, find unity gain with your loop. Set loop to "FSTW". Connect a cable from send > return and adjust the send level until there is no volume change when footswitching the loop on/off. This will be ~ 1:30 on the loop send level depending on how hot your V4 preamp tube is. Once you find the spot, leave it there.
If your DR is a 3-channel or a later 2-channel, you have a parallel loop with a knob labelled "mix". Turn that all the way down for now. Connect your FX in the loop: Send > FX input > FX out > loop return. If your FX has an input level adjustment, set it halfway up. If it has an output level set it at 1:30 (7 on a 10 scale or whatever). Whatever kind of blend/mix/wet-dry adjustment it has turn that all the way up.
Set your amp's Output to ~12:00 for now, and turn the channel masters all the way down. Footswitch the loop off. Starting w/ the clean channel, bring the channel master up to your desired volume. Now switch the loop on and turn the FX Mix on the amp up until you get the desired ammount of effect. If you are using pedal-type effects, you can overload them with too-hot a setting on the channel master, so if you need more volume, Output up/Master down until it's loud enough without overloading your pedals. With rack-type processors set their levels to +4dB (or "pro" or whatever). Balance the Channel Masters on the other channels to match vol with your clean.
Once you have the hottest clean channel level possible without clipping the FX unit, and the other channels balanced accordingly, leave those settings alone and turn the amp up/down using the Output and Solo controls. Adjust the desired FX mix with (you guessed it) the FX Mix knob on the amp. Unless you're after some extreme sounds for some special purpose, lower settings usually sound better.
