Make sure that whatever you put in the loop, can take what the front end of the amp puts out. Many pedals are designed to take an instrument level signal, instead of line level (which is what my amps loop has). So check that. When used properly, a good EQ pedal good very well be the key here.
I also have an MI Audio Boost n' buff, which can also work in the loop for this purpose. Especially if you run it at 18 volts, it has a very good headroom and can be used in loops too. I haven't run it like that though, I have a second master volume on my amp. However, I concur what was said about EQ, and I'm very tempted now to try an eq in the loop, with a slight boost in the mids.
-Zaned