I have tried using a Boss DD-3 in the effects loop but I can hear a significant drop in volume and tone when the loop is engaged. If I remove the pedal and just have a patch cable going from the send to return, there is no volume or tone loss when the loop is engaged. If I move the DD-3 to the front of the amp there is also no volume or tone loss. Why can't I use this pedal in the effects loop?
If it sounds OK with a straight through cable (this is most likely the same signal path as having no cable), then the issue is probably with impedance of the pedal. It may be you need a buffer, or another pedal that can act as a buffer by having a different impedance.
The front guitar input will have a different impedance to the FX loop and thats why it sounds OK plugged in there.
Its a huge subject I suggest you try to Google "guitar pedal impedance in fx loop". There is lots of info about your exact problem.
Hope this helps.