I thought we had the larger Acton, but it's actually the first gen smaller Marshall Kilburn. It's about the same size as the AM4, obviously thicker tho.
I ran a short EBS flat 2x TS -> TRS -> 6.3mm to 3.5mm adapter -> Kilburn Aux in from the Fractal AM4 outputs. A longer cable would be preferable, I could barely make this reach!
Works just fine in stereo, even has some spread with e.g ping pong delay.
It gets plenty loud when it's sitting on a table. It even has a surprising amount of bass on tap where I actually used the bass knob on the Kilburn to reduce some of that when using my 8-string guitar.
Obviously it won't hold a candle to proper larger speakers, but as a home volume solution it actually sounds pretty good!
EDIT: You can even use the AM4 headphone out to connect to the Aux in. I think it sounds somehow a little bit different than the main outs, but I can't put my finger on it. But it's a more practical way to connect if you don't have a longer spare 2xTS -> TRS cable lying around.