Should I use "闷声发大财" or "闷声大发财"？
Here is the summary of google hits on each term, including both Simplified and Traditional Chinese. Blue is for 发大财, and red is for 大发财:

So apparently 闷声发大财 is more ubiquitous, both in Simp. and Trad. Chinese.
But it is not a problem of correctness. Nothing is grammatically wrong in either form. There might be something slightly off for 大发财 because it doesn't conform to similar phrases like 吃大亏, 撞大运, etc., but it's not that important. The thing is, even if 大发财 was indeed the original form, 发大财 has already become an internet meme, and there is no way to stop that.
After careful listening, I think there is not doubt that the Chairman did say 闷声大发财. The pronunciation of 发 is hard to dismiss if you know how 入声 works.
However, there are like 1 billion native 官话(Mandarin in the broader sense) speakers who can't distinguish 入声. Also verb + 大 + object has always been a prevalent form. These could be why the 闷声发大财 version gained it's popularity.