As a first step, try unplugging the input signal
leads from the amp ... if you can still hear hissing and popping .. the
amp is at fault and will need to come back to us.
Try swapping the signal leads over, A for B, B for A ... has the popping moved to the other channel? if so, its probably the input signal.
Try switching the amp into "parallel" mode (the middle position of the switch on the back)
This will now take the signal from the CH A input and feed it to CH A and CH B. If the popping remains, it is a fault with the amp, and will need to come back to us.
If it goes away when you switch to parallel mode, try plugging the CH B signal into CH A and see if it comes back.
If it does come back, then the clicks and pops are coming from the effects unit/modeller, you'll need to speak to them. You may be able to confirm this with headphones, if you can plug to the same signal out.
If it does not, then its probably the amp input circuit ... try switching the amp back to stereo mode, and plugging the known good "A" signal into the channel B input, if it sounds poppy and crackly, its something in the amp.