Amazing. We were annoyed by this at nearly the same time. I noticed the same annoyance in the Essex A30, both Matchless channels, and the Fawn models the most. Which, annoyingly, are the types of amps I like the sound of most. The artifact is almost like the radio transmission distortion while the Rebel alliance are in their attack on the Death Star. Why would anybody want this? I found it incredibly frustrating to have to hear it. And it seems very pronounced in these models in particular. Especially when playing a high A (plus or minus a few 1/2 steps). I was getting the noise via all outputs: Fender FR-12 via a Walrus Audio Canvas and via a mixer connected to the line output of a Blackstar Amped 1 (no cab sim) into headphones. It just didn't matter. And I wasn't getting the remnant/distortion from my Iridium. I thought I had a bad Stomp XL.
So... if you happen upon this thread... turn down the Ripple on the last page (or two) of the amp block's settings to get rid of that annoying 'remnant' or distortion. It's simulated ripple from the power filter circuit.