If your parallel loop has a mix pot which allows the full signal to be sent through the loop, rather than just a fraction of it, then you can use the decimator. It needs to be pretty close to 100% to get the most out of the ISP Prorack. You are effectively using the parallel loop as a series loop in that case. The ISP would still work to some degree because it has one gate that works at the input stage, and the 2nd gate works on whatever signal is passing through the loop. If your loop send mix wasn't able to send the full 100%, say 90-95%, it would probably still be good enough. Anything less than 90% would allow noise to pass through and then you have to consider how noisy your amp is in general. Some amps are quiet enough that having 10, 20, 30 or even up to 50% of signal being un-gated would still be ok.
To work out how much you can send through your loop. Put a straight lead from send to return (no FX), then set the send to maximum. If you get very little sound, or none at all, then you are running close to a series loop anyway. If it's still pretty loud, the ISP might not be worth the money.
It's not a difficult mod to convert a parallel loop into a series loop though. About 5 minutes with a soldering iron. It's also fully reversible. Might be worth considering since many digital effects are designed for full series loops anyway.
