I use iZotope Ozone 5 for mastering. It has a great volume maximizer and that is only one of the 6 “effects” that you have. You get all kinds of controlling over the sound that I just don’t have time to write here about. But getting -6dB Integrated Volume without any distortion is very easy.
It’s not a cheap software but it will drastically change the sound of a song when used correctly. Send me a song and I will demonstrate: firstname.lastname@example.org