Hi, I'm sure Gary wouldn't mind me posting this email when I asked him about running a Roland GT6B in the effects loop. Hope it's helpful.
The key to using effects through any effects loop is to get the levels balanced correctly. If your are getting fuzz or distortion, the signal going into the effects box is too big and overloading it, or the signal coming out of the effect is too big and is overloading the effects return op-amp in the 800. This is similar to a guitar amp front end being overloaded to get a distorted sound, and the master volume being used to control how loud the distorted sound is at the speaker.
The series loop simply breaks the signal going to the power amp of the 800, and there is no volume blending provided by the 800 when using this loop, or any series loop for that matter. Any blending has to occur in the effects unit itself. If the effects unit does not have the ability to blend the dry with the processed signal, it will not work. Note that once you plug something into the series effects send jack, the entire preamp signal is interrupted. This is why it is called a series loop.
The parallel loop gives more flexibility in that a portion of the dry signal always appears at the effects return circuit even if nothing is plugged into the parallel effects send jack. Give it a try and see that the amp still works when something is plugged into the parallel send, thus a parallel loop. This allows a simple blending of the dry and processed signal just by controlling how loud the return signal from the effect is. Both the dry and processed signals appear at the same time.
We have to distinguish between noise and distortion. Distortion is fuzzed out signal due to overloading and/or levels being set improperly. Noise can be hum, hiss, or some combination of both. Hiss can be generated by improperly impedance matched signal levels, or simply due to noisy parts in the electronic circuit (historically, effects boxes are not designed to be ultra quiet given the fact that they are cost sensitive devices and their intention for use is with lead guitar players who consider system noise to be a positive thing). Hum is generated by ground loops which are created when any two pieces of electronic equipment are connected together which are run from the AC power line. Battery operated effects boxes do not create ground loops when run from the battery alone.
All in all, you have to experiment with anything put into the loop to get the levels set properly and to keep the hum and hiss to a minimum. There will be certain effects that are noisy and some that won't be. Some will work better than others. The effects loops of the iAmps offer high input impedance and low output impedance which is the best scenario possible. I could make any effect work perfectly with the 800 by tweaking circuitry in the amp and/or the effect, but this approach would be tedious. By the way, the problem you are having with the Roland GT6B is all related to signal levels. In almost all cases, the signal level is too big going into the effect, or leaving it.
