Yeah, the proper way is to run it in your FX loop. It's not designed to handle speaker level signals; the nominal input level is only -10 dBu, so that would be a baaaad place to put it!
They tend to "enhance" the highs and lows, while dropping the midrange. I'm a huge fan of midrange, so I dislike them. Also, the tone always seems to have some sort of digital-style thing going on when you use them. Just my opinion, of course.
I believe these were originally designed to restore signal structure that was lost from either long cable runs or over processing. Whatever the reason they were designed I would use it sparingly, as they tend to color your tone. The correct way to use it would be between the Preamp/processor and the power amp.
In the loop with both knobs almost all the way off! They can be a delightful addition to some rigs if used sparingly, I do HATE them with mesas though!