Actually, a huge number of users have multi FX. There are a variety of FX levels, so usually the channel master or amp master volume controls the send volume.
If your pedal is not being overdriven, then the loop level is not actually line level. Also, a jump in volume when inserting a pedal is also not about the send level.
Either the pedal is not unity gain, so turn down the pedal output level, or the loop adds gain when you plug into it. You can check if the loop is adding gain by plugging a patch cable from fx send to fx return. If that causes a volume bump, then the loop has gain. If not, it's your pedal.
Elvis is exactly right.
Though he forgot to mention that effects loops are the spawn of satan.
These pedals worked fine with Marshall, Mesa and Orange amps.... somethings off with the FX loop on the MT-15. Is it Parallel or serial?
Parallel vs serial won't give you a volume jump like that unless the pedal has gain.
Divide and conquer.
Pull the pedals out off the loop.
Play with nothing in the loop.
Put a patch cable between FX send and return. Same?
Add ONE pedal at a time.
When you find a volume jump, stop. That thing is the cause. Then you have to figure out why.
Does the BF2 cause a volume jump between on and bypass? Does it do the same if you plug into the front of the amp on the clean channel?
I've messed around with my setup on the weekend and i can isolate it to the Flanger. For some reason the loop doesn't agree with it so i might need to run it out front or look at another brand.