Mixdown is quiet compared to other audio sources.

Yep... ran into this myself early on. 

The mix sounds good and loud,,,, nice and full in the DAW playback but when you export, the level is low.

You go back and double check... and all the outputs are bumping the red so you wonder what happened. Why is the exported mix not as loud?

Well, what I do.... and I'm not sure this is the "right way" to do things or not but it's what I do. 

Step 1: 

Before I finish the mix down, I insert Ozone 4 early in the recording process. This is a mastering plug in suite. Folks say that cake contains all the plugs needed to master the music..... and I know it does. The problem is that most folks do not know enough to get the maximum out of the cake plugs to PROPERLY master the music. 

note: small "m" mastering.... means polish the tracks, get the most volume out of them in a reasonable manner, make them sound better.

EQ, and compression, as well as some reverb and maybe a limiter are the main components I use. Ozone has them all (and more) and has presets to halfway get you started in the right direction. I always tweeze and change the presets. I save my new presets. 

Doing this small "m" mastering will bump the volume a few to several DB up as well as add some shine to the music. 

Step 2: 

I export the song out of X1 and reopen it with an external wave editor such as NCH Wavepad. I use this editor to look at the wave and trim the count in and the end to the exact length I need. Then, while still working with the wave, I apply the 'Normalize" function in Wavepad to bring the highest peak to 97% of 0db. That gives me the most volume (short of going above 100%) before I convert to MP3 to post online. 

People have commented that my music is sufficiently loud, even the acoustic stuff, and looking at the waves, there is NO evidence of clipped wave forms. 

This song.... on my sound click site currently, is a classic example of the output of my process. There is only one lone "spike" near the end which even comes close to the 97% level. Yet the song is full sounding and compares favorably to anything you hear on the radio with respect to volume. It's not necessary to crush and squeeze a wave with compressors to get it to the proper level.  

Footsteps: http://soundclick.com/share.cfm?id=12048139

It's usually recommended to use subtle compression on tracks or buses, too, so you don't need to squash so much in the Master Bus.

It's also common to put in the Master Bus a high pass EQ around 40 Hz  to get rid of unnecessary low rumble, which is practically inaudible but adds a lot to the RMS-level.

if wave pad made the wave smaller, you were already louder than the 0db point. 

can you post a picture of the exported raw wave as it appears initially when opened in wavepad? 

also post a screen shot of the DAW screen with say, 4 or 5 audio tracks filling the screen? 

I'm curious.

My exports are always less than the result of the 97% normalization in wavepad. Some do not increase by much but some do so noticeably.