There are tons of tricks to get more apparent loudness even on fully slammed, ready made, masters.
One of them is low level upwards expansion in combination with relatively aggressive saturation (the nice harmonic kind). Now squash this parallel track, EQ out bits that eat up too much room, smash it to bits with a limiter and sneak it under the original audio.
Add to this trick a bunch of tactical boosts in the 1-8kHz region, only on the side channels and you get some more added energy again. You can also try saturating the sides a tiny bit more than the mids.
Now you'll need to add another limiter of course to keep the ceiling in check.
I've tried all kinds of these tricks on already super loud master CDs and successfully gotten them considerably louder. Of course in my opinion they all turned to absolute crap but it seems that a layman can not really tell the difference, except that one is louder. I even got some comments that one is way "phatter".
In short: To achieve this you have to drop the typical mastering engineer mentality and just go for broke. There are so many tricks to getting things louder than just the traditional ways. Traditional meaning gain staging in analogue gear, creating the correct frequency balance and then limiting/clipping. It's just that most of the other methods make us cringe and want to hang ourselves.