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!!!

The loop isn't buffered like in many (most?) modern amps. 

While the Kleinulator is a dedicated buffer for the exact purpose of effects loop use, there are a few cheaper options, such as running Boss LS-2 Line Selector 9used, under $50 on eBay) or a now OOP DOD FX30-B Gate/Loop (these things can be had for as cheap as $10 on ebay).

Just place your effects in either pedals loops and leave that loop turned on all the time, the Line/Loop pedals themselves are buffered in/out and the Boss even adds a level control, which I think would help a lot to dial in the level's you want.

What would you do if the amp did NOT have a passive effect loop?

And this is not just dealy, but also chorus, flanging, phasing, correct?