Title says it all. Preamp tube for V5 is fine. There’s volume loss no matter what I do with plugging in cables and whatnot into the input jacks for the loop.
Mark sure the cables work and are correctly placed and the effect you're using has power.
If it still doesnt work, try using a small cable and go from send to return and see if it is still low volume. If it is, check the appropriate preamp tube. If the tube is okay, try cleaning the jacks.