I am trying to find a compilation of stories that was edited by Griffin or Griffith possibly. Cover was maroon. From the 1990s I think.
Contained so many great stories including:
The Omnibus of Science Fiction by Groff Conklin was published in 1952 and has candidates for at least two of the stories you describe. It was reprinted, among other times, in 1980 and 1984. The dust jacket of the Bonanza edition of 1980 has a sort of dark orange background to the front cover artwork; the inner binding of some other editions is primary red.
In this book, the metal cube story would be John Thomas's Cube by John Leimert. A boy finds something in the front yard that wasn't there before.
“But, Mother,” he said, “it’s the queerest thing I’ve found. A little block of metal so heavy I can’t lift it. Come and see. Please, Mother.”
The cube cannot be moved; even when soil is removed from beneath it, it hangs in the air exactly where found. Eventually...
(his mother said) "I wish that block would take itself off to whatever place it came from.” “So do I,” John Thomas said. “I’m tired of it.” At that precise moment there was a shout from the yard. “It’s gone. The block has gone.”
The "one about astronauts who visit a world and ascend gradually to higher levels" is Environment by Chester S. Geier, described in the answer to this other question. In summary, two astronauts learn things by pictures in various buildings on a deserted planet, and eventually become beings of pure energy.
I do not have a copy of the book and cannot identify the third story.
