tlw
Another factor in "loudness" is how we perceive volume.

Our brains tend to judge volume not by transient, brief peaks of a few milliseconds but by it's average over a longer (though still brief) time. This is why we distinguish between peak volume (the absolute maximum reached) and RMS volume which (put simply) is a kind of average. Pulling up the transient peaks will have less effect on perceived volume than raising the RMS volume.

An example of this is the Fuzz Face pedal. With its' volume and fuzz controls maxed out it sounds deafening compared to the clean amp volume, yet the Fuzz Face design can generally only just reach unity volume with the "clean" sound if you look at what it does using a meter. It sounds louder because it compresses like mad and so raises the RMS level which makes it sound really loud even when its' peak volume is actually lower than the clean signal. Many a guitarist has found that the setting on a fuzz, distortion or compressor that sounds like a huge volume boost at home results in their relative volume dropping when in the context of a band mix.

So to maximise volume we need to both raise the transient peaks as far as possible and also reduce the gap between peak and RMS by raising the RMS level. This is done using compressors or limiters, usually after mixing and at the mastering stage. A good starting point is to set the master bus's meter to read both peak and RMS. then put a limiter last on the master bus (out of Cake's plugins the Concrete Limiter is particularly good for this, though Boost 11 or any compressor capable of a very high compression ratio will do). Set the limiter up with a high ratio and adjust the output so that the maximum volume never goes above 0.3 dB or there about. Now use the threshold and ratio controls to raise the average level.

Go too far with this and your music will certainly sound loud, but lose a lot of life and the punch that comes from quiet and loud transients. An RMS level somewhere between -20 and -12 seems to be the popular area to aim for, though some commercial recordings are pretty much a brick-like transient-free block if you look at their waveforms....

And you most certainly do not need Pro Tools to do this. Sonar is quite capable of doing the job as are Cubase, Logic, Soundforge, Audacity, Wave Lab, Reaper, Studio 1 and any other DAWs or sample processors that either have the functions built in or can run plugins.

Oh, and if you're mastering for vinyl forget all the above. Vinyl has technical requirements that mean simply maxing out volume is a very bad idea indeed. Which might be why some people prefer vinyl to CD, what they are hearing in the vinyl (which on paper is a worse medium) is the transients lost in a digital master which focused on nothing but getting it as loud as possible so it grabs people's attention (as every good hi-fi sales person knows, if you turn up the system you particularly want to sell a little more than the one demonstrated before it, the odds are the customer will think the louder one sounds "better").

Sanderxpander
I don't think the Concrete Limiter is included with X3P, that's why I mentioned Boost11. There's also the BlueTubes brickwall limiter.