This is ripped from Soldano's website:
"A series loop interrupts the signal path between the preamp and the power amp and inserts the effect processor signal into that path...It’s basically a one-lane road going from one place to another...A parallel loop offers two paths from the preamp to the power amp. One path is a direct connection from the preamp to the power amp as if the amp had no loop at all. The other path sends the preamp signal to the effect processor (via the loop) and then routes it back to the power amp, mixing it with the direct (dry) signal.
A parallel effect loop is a good thing and the ability to be able to adjust the mix is nice. However, they don’t work well if you are using effects that change the volume of the signal (such as tremolo, compression, or noise gates), or when mixing the wet and dry signals causes an out of phase situation. Technically, if you turn the mix to 100% in a parallel effect loop, it should operate exactly like a series loop, although this is not the case with all amplifiers on the market."
I have a Recto Recording Preamp, which has the parallel fx loop and had the same problem. In the manual, Mesa stated that they purposefully do not allow for the dry signal to be eliminated with the mix knob, but I don't remember their reasoning or if they even provided one. I didn't have to get the mod since I ended up just putting the post effects from the FX8 between my Recto preamp and my Rectifier poweramp.