Question [noun + が + [TRANSITIVE verb (て-form)] + あります] vs [noun + を + [TRANSITIVE verb (て-form)] + あります]

My Minna textbook describes the following construction: "the sentence pattern [noun + が + [TRANSITIVE verb (て-form)] + あります] is used to indicate that a situation exists as a result of someone's deliberate action".

In a subsequent exercise, I noted the following two sentences:

Could 箱 have been construed with が rather than as a direct object with を? What difference in meaning is there either way?

Thanks!

You could, with similar meaning.

玄関に箱を置いてありますね = "(subject) has put a box in the entryway, haven't they?" Subject often being the listener so, "You've put a box in the entryway, haven't you?"
This sentence is about the person who put the box there, and we probably know who it is (although だれか could be the subject).

玄関に箱が置いてありますね = "A box has been put in the entryway, hasn't it?" Subject of this sentence is the box itself, which implies we don't care and may not know who put it there.

A better natural translation will often be that a box "was left in" or "is in" the entryway, but those translations won't exactly mirror how the grammar works.

Ah, I get it. The subject will always be the focal point in this construction, even if not explicitly expressed. That nuance went totally over my head. Thank you.

It's said that を emphasizes that the action was done previously for a purpose comparing to が, so the nuance is close to ～ておく as a result.
e.g.
裏口の鍵を開けてあるので（＝開けておいたので）、そこから入ってください。