Wall of sound is the term I and some reviewers use to describe a flat and wide soundstage, like everything is on a flat surface without much sense of distance and layering. I personally dislike it, but then I know some who actively seeks that, so YMMV.

I haven’t heard the Atom so I can’t comment. I mostly review IEMs so I generally don’t pay much attention to desktop gear. A Fiio K7 is more than enough (though, again, a fellow IEM reviewer insists me to upgrade to K9).

In my experience, just grab a proper desktop setup (the one with external power supply rather than USB-C from laptop) from a reliable brand like Fiio, Schiit, Topping, etc., and you are done. Some people want to spend many thousands on these. Not me. An all in one like K7 or the Schiit stack is enough for me.