I had the same problem when I received mine and found this thread / forum when searching for solutions. For anyone else having this problem, gain staging solved it. If you're replacing / adding to your existing set up, you will need to approach things differently. I was using a joyo drive pedal + 2 amp emulation pedals into an interface before. The levels that worked then didn't work with the 2 notes in the loop. Here's what I had to change:
1) If you're coming in from a guitar / bass or a guitar pedal, make sure the switch on the right hand side is changed from 0db over to the +12db guitar position on the right. This got rid of about 75% of the hiss for me.
2) You're probably coming out of the cab m+ hotter than you were before so if you had the gain on your interfaced turned up, turn it down. Before I had to have it around 11 o'clock when I was feeding it an amp sim pedal. With the two notes I had to turn it all the way down to zero. This took the hiss down another 20%
3) Even after this it was still coming into my interface hot and causing clipping. Depending on the patch I had to take the output down from 3 to 6db.
4) Any last bit of hiss was taken care of by setting the gate.
Anyway, I hope this can help anyone looking for solutions to the hiss problem. Doing this took the cab m+ from "the hiss is so bad I'm going to have to return this" to "wow this thing gives me the sound I hear in my head".
