Just to add a bit, you may need to adjust audio buffer/sample size to fit your need, too small buffer size will cause jitter issue (the crackling and popping noise) while buffer that's too big increase latency. You can reduce the latency by increase the sample rate (I think your Scarlett support 96khz). Check on your mainstage setting if I/O safety buffer is ticked.
Check out this link, see if it is helpful
https://support.apple.com/en-us/HT202982
Good luck and hopefully you can enjoy playing soon
Re the crackling, I am not familiar with Mac, but I run a component based mixer with the engine running on a PC. It is important to turn off everything that might interrupt, or set them to a lower priority. For example. I had a click every few seconds that turned out to be the computer checking to see if there was a CD or DVD in the drive. Things you might not think of.