It says "open other end, this end up", I have no idea how that could possibly be done.
Tip it onto its end and open the bottom. Flip the flaps out of the way and sit in back onto its base. Lift off carton.
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Or cut the bottom flaps off with a box knife and lift off. Wouldn't want to disobey the box. Unless the box also has a picture of a knife with a line through it...
Presumably you've spent enough time with it to realise why just lifting it out isn't much fun (I know this from experience...)
I'd try to keep the box intact in case you have to return it or something. You won't hurt anything if the box is upside down though. I'm sure the tubes are wrapped up etc. I'd try opening the top and then setting it on its side and sliding it out (while someone else holds the box potentially).
Now I have bigger problem, brothers and sisters: there is continuous, very loud hissing when I turn on vibrato, and I need to return it, hence, how do I put it back into the box? ...

The box is intact.
When I opened it, I flipped it sideways to open the bottom, then flipped it back, then lifted up the box.
It's a major project, but I've made it!

1. Fix the foams with tapes on the top of the box.
2. Put foams below the amp (some heavy lifting here)
3. Put the box on the amp, like a hat.
4. Flip sideways and tape the bottom.
5. Flip back in the upright position.
never new un-boxing and boxing were that complicated
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Damn dude.

Now you've got it all boxed up again and can't trouble-shoot the problem. Did you already contact the store? The hiss could have been any number of things that you could have identified and possibly fixed on your own without sending back. Tubes. Reverb connection. Guitar insert. Cable. Microphonics. Distance and position of amp.

Not saying it doesn't need to go back but at least try all the basic things first. We are here to help man.
Quote by diabolical
See if you can reseat the tubes before you return.

...but, if they don't know how to open a box, isn't this a little complicated?
Quote by SpiderM
...but, if they don't know how to open a box, isn't this a little complicated?

Repeat as necessary.
“Ignorance more frequently begets confidence than does knowledge.”
Charles Darwin