New user, technically inept!!

I have JTM 45 head with an effects loop into which I tried running standard stompbox pedals (like your standard BOSS overdrive, for example). However, I experienced a dramatic loss in volume and a heavily distorted sound. I email Nik and he told me that the loop is 'passive' and runs at 'line level,' so the levels between the loop and the effects are mismatched.

The research I've done leads me to believe that I must run rack-mounted effects in this loop, but before I begin to look into that I wanted to know if there is anything I can do to lower the level from the loop 'send' down to instrument level and run through the desired effects, and then bump it back up to line level for the loop 'return.'

Any help would be EXTREMELY appreciated!!!