I've been lucky, the only time I get a "popping" is if I change wattage on a channel when the amp is not in standby. If changing tubes doesn't fix your hiss, a simple fix might be to get a noise gate pedal in the loop.
Welcome! I'm new here to and I recently bought the 5:50+ too. The clean channel on this amp is amazing! Absolutely perfect in my opinion. I think you're going to love this amp. Congrats my man!
I had the same dilemma but decided on the 5:50+ because of the amazing clean tones. I can always add pedals to get a great crunch/lead tone, but you can't add a pedal for a great clean. The 5:50+ is an amazing amp.
I was in the same boat as you. I wanted something with a nice tight low end and in all honesty something more "American" sounding after purchasing and returning a Marshall SL5 combo. After watching some youtube videos and reading a bunch of reviews, I ended up purchasing the Express Plus 5:50...