I got it in the mail today. Opened it and set it up for 8 Ohms to mach the input of my 4x12 Cab. Mono input. Plugged my guitar in straight. Started playing for a bit and I start hearing loud crackling sporadically on the clean and distortion channels as well as a loud thump sound before I turned it off. Any ideas what the issue might be?

Thanks - Calvin
This is I'm sure a very dumb question but, does that mean I just pull them out and put them back in or are there any special steps to it? Sorry for my inexperience.
That's exactly what you do. Wiggle side to side a bit if they're tight. You're just making sure that they're snug in their sockets so you don't have to pull them all the way out if you don't want to.