I would not attempt to balance the SPL levels from a small speaker compared to your mains. The small speaker won't even come close to the sort of SPL levels you can get from your main speakers. That is not the purpose of them. That is missing the point. The whole idea is to listen to your mix summed to mono down at low level on your small speaker. (note one small speaker as well) That is when they do their best work and reveal the most.
 
If you have more than one small speaker (eg different brands, not two channels) then it is not a bad idea to balance the SPL levels between them so there is no major leap in level when switching between them.

I read Stevie Wonder used to hook the output of a perspective mix to an FM  modulator and broadcast it to car radios in the parking lot to hear the results of a typical car radio sound and how the mix sat in that environment.
 
BTW I know its not the same, but my Fosucrite DSP24 has the VRM with a set of "cubes" as one of the models..

Interesting discussion.
Good info.
I may just get one instead of 2 after reading this.
 

This sounds like a very useful mixing tool.  I have been researching these for the past few days and plan to buy a single active mixcube.
Thanks for the info and the great follow up advice!

Just ordered a pair off of Musicians Friend - I am splitting the order with my brother, so we will both end up with one.  There is currently an additional $75.00 off if you order over the phone and mention the "Exclusive for Phone Order Sale".  I think the sale ends real soon.

Hey guys, 
 
I'm no engineer at all and Have a limited budget but I was able to compensate by using good references and soundforge to make my mixes good even though I monitored with just Huge PA speakers. Well i thought I was being smart by selling those big bulky things and buying some inexpensive TAPCO's 5. Every since my mixes have been horrifically muddy and way too much low end. I usually can adjust but I can't because these damn tapco's make things sound TOO good. My mixes would sound just like my reference until i take them else where. Would a mixcube help with low end problems even though it won't play the extremes?

http://www.youtube.com/watch?v=eLqL8TBD6LU
 
I know this is an older thread...but I just did a video of my impressions of the Auratone speakers I use.
Take it with a grain of salt, as everyone has an opinion!  :-)

The modern day version of these speakers do a fantastic job and do the same thing very well. I am a user of the small speaker and believe they are very very useful for mixing on. You don't even need an expensive Avantone speakers either. I have just got a small powerful driver in small box and it also works very very well. Just as well as the original Auratones.

The speakers in these small boxes are not full range drivers at all. They have a limited low and high end. It is the mid range that exposes everything in your mixes so much.

Another thing, I believe it is better to not use two of these and switch them into mono. It is better to use one only and feed a L+R signal into it instead. And keep the volume well down while you are listening to them as well. Sit right in front of it. The reason why one works so well is it shows any mono compatibility issues straight up and is also great for preventing things that you may have panned extreme left or right from getting lost in your mix. As the mono sum signal tends to push sounds that are panned centre up a little. If you are loosing the extreme panned parts then all you have to do is just nudge them a tad so they become audible again in the small speaker. Then when you go back up to your main speakers again you will find not much has changed there with those parts. But now you have satisfied two speaker systems rather than just one.

They are not just for mixes that end up on TV's etc. They are very effective for hearing how your mix will sound on a full range system. They are fantastic for dealing with dense mixes with lots going on. They are great for setting vocal levels correctly over the music. Things that are even just slightly loud in you mains too like a snare that might be just a tad loud, and you wont hear it so much on the mains, tends to leap out more on the small speaker.
 
You won't get an original Auratone easily or cheaply and it will more than likely have been destroyed over many years of use. Don't waste your time trying to get one, it is unnecessary.
 
You still need to check your mixes up loud on your mains though primarily for bass and reverbs. You wont hear them very well on the small speaker. Saying that though if you do set reverb so that it is barely audible on the small speaker it is often just right on your mains. Buy your mains will always let you hear reverbs a little louder. There is some ground where you can get reverb barely audible on your mains and just hear it too on the little speaker. You have to juggle that slightly I have found.

I tend to disagree a little bit with the above, in that the Auratones are easily available and that they became popular as a "microscope" for the midrange, versus showing how your mixes translate on lousy speakers.  For the fact that TV stations used them to compare their sound over TV speakers of the day, I think led many to believe that was the reason for their popularity, but I think top engineers in studios made them more "famous" for the reason I describe above.  I also agree with that reason.  Therefore, I think many went out and got cheapie speakers to simulate, ...even the Radio Shack Optimus series...but I think they all fell short of what the Auratones would do in revealing midrange balances.
 The Avatones have a wider freq response, and therefore diminish that "microscopic" view, that I am describing.  I think I covered much of that in the video, but I'm not the best in describing things sometimes.
Thanks for the Youtube comment too, btw.  I also responded as well.

Oh...they actually are "fullrange" too...but have steep roll offs, that would make them seem like they were not.  But I think our definitions of "fullrange" are a little different as well... where in the video I compared them to a 3 way speaker, where each speaker was NOT intended to be "fullrange" unlike the single driver Auratones.