You can't do more than -0dbfs. Not sure if that's what you mean by your db values. There's no secret or magic to reaching -0dbfs. You shouldn't go there either because you can still clip, even with a limiter.
If you're mixing and insist on exporting to wav to apply more processing (master), I'd suggest just applying the processors on the master bus unless you're running out of processing power.
The point of mastering is to take a mix and make it sound better. That should be your first focus. If you're doing your own mastering and you can improve your sound during the mastering stage, this only means that you could have done a better job during the mixing stage. If you have a snare that you are taming during mastering, it means that it's too loud on the mix. So, just go back to the mix and take care of it there. Then start your mastering stage again and again unless you find that there's nothing to fix.
Now that your mix is really well balanced you can limit the crap out of it for the sake of loudness. Now that I condone this, but we all know that this is what everyone wants.
Your mix should not be at, or close to 0dbfs. If it is, you mixed too loud. Go back and fix that.
Oh, yeah, if you need more volume during the mixing stage, use your volume knob. That's what it's for. Don't pull up the faders, use compressors, or limiters for the sake of loudness. If you did anything for the sake of loudness during the mixing stage, remove it and start over again.
Okay, so now you have your mix right? Great. That's what we want. Apply your limiter on the master track and see how long you can push it before it starts pumping. If you are not limiting much, it's not loud, and it's starting to sound like crap, this means that your mix still isn't right. A good mix will stand up to a lot of punishment. This is how commercial tracks handle all that limiting so that they can be loud. If your mix sounds fine, maybe the problem is what doesn't make a sound. Sub-bass may not be audible to you but your limiter doesn't care that you can't hear it, because it can hear it. You may or may not have to cut off some of the inaudible low frequencies. At this point, what you have to fix should be more obvious.
Multiband compressors are used to fix bad mixes. Fix your mix and forget about the MBC.