Do you experience the same crackling through headphones if you connect them to the Scarlett Solo?
If not, then the issue may relate to the outputs on the Scarlett Solo being unbalanced. I note that the outputs of the 0404 USB
are balanced, and so if you're using balanced cables between the interface and the speakers, this will resolve any noise in your signal caused by external devices etc.
Because the outputs of the Solo are unbalanced by design, if there are some electronic issues in the vicinity of the Solo, this could cause crackling and buzz through your speakers (which would not be heard through the headphones).
If this is the case, then you may wish to consider swapping the Solo for a 2i2, which has balanced outputs.
Please let me know how you get on.
Simon // Focusrite Technical Support