Having worked extensively with a a JamMan, I can tell you that it does respond best when patched into a parallel aux send/ret (ir even better returned to it's own channel on a mixer!)
For some reason the inputs on it are, shall I say "sensitive" and it is very easy to overdrive the inputs which is not a pretty sound at all!
Running the JamMan in series from the dbx MAY do this......
Currently, I run my JamMan in the aux loop of a Raven Labs MDB1 mixer/DI/Preamp, and the results are great. I run that signal to a Raven Labs PHA1, which is set flat and a TC Mo One is inserted into it's aux loop...that is sent on to a power amp.
I have found, that when using loops, it is best to look at your amplification more as a PA set up. All of your coloration should come before the looper, the fx, if any, can also go before, but I prefer to "treat" the loops with fx. You should have ample power, headroom and a fairly transparent sound.
Bob Sellon, the man who designed the JamMan, recently did a mod/upgrade to mine which now puts it in stereo, allows for multiple parallel loops and some other neat tricks, so I have started running it from an aux send of a little Behringer 802 mixer, returning the JamMan's stereo out to their own mixer channels, and running the whole rig in stereo...which is awesome.
Suggestions? Without going out and buying a little mixer (tho I really recommend the Raven Labs MDB1...three channels so you can run different basses, each set up for it's own "personality...incredibly clear and transparent tone, and a great DI to boot!), have you tried running your bass straight into the dbx and putting the JamMan into the fx loop. Since SWR uses a parallel fx send/ret this would give the highest sound quality/lowest noise with your setup.
Just a question..how much "squeeze" do you apply with the dbx? Is it really neccessary? You might find that using more manual control of your dynamics can allow you one less thing in your sig chain. I say this only 'cos everytime you plug into some device it effects your tone...somehow altering the tone of your bass. Sometines this is a beneficial thing, but usually not. That being said, I believe the input on the dbx is a fairly low impedence, line level input, which will not compromise things as much as an instrument level input, but, since you have been placing it in the aux loop, I am assuming you have found the tone to be better when not run direct.