Don't ask "have been," but ask "might have", Or you use "may have" with a past participle when suggesting something happened or was true, giving possible explanation
Don't ask "have been", but ask "might have"-----
"might" is a modal verb used with the basic form of a verb.
You use "might have" with a past participle to indicate possible something happened,true,possibly explanation for something;eg:-I heard what "might have been" an explosion in Convention Center HK.....
Hence:-modal,pragmatic on "might have" ;-eg:-
On rearranging a sentence,we've:-
The most wonderful moment in his life "might have been":-
(1 ) getting married;
(2) promote to a high official post by the Emperor
(3) won a prize, have a windfall
(4) joining a party;
might have be--------with a present participle
might have been-----with a past participle
It is to indicate that it is possible that something happened or was true with a possible explanation.
Hence on rearranging the sentence, we've:-
have been--->might have been
eg:-This might have been ----->the most wonderful moment in his life.