...just a quick thought. Is it that you're getting different levels on different systems at certain frequencies (instruments, if you like) in your mix, or is it the entire mix that seems to change in level?
I've been setting up a little studio in my new place and have become marginally obsessed with room acoustics. From this I'm wondering whether it's a possibility that your room is doing this to you(?). (When you listen on different systems, I'm assuming they're not in your studio...) What you might try is to see whether different CDs played in your studio have similar balance problems, focused around similar frequencies. If so, then the problem could very well be your room. If you Google around a bit on the subject of room tuning or acoustic treatment you'll be amazed at some of the stories -- in some cases, even the smallest investment (DIY) and attention paid to room treatment seems to make all the difference in the world.
Just crossed my mind, maybe meaningless.
The other possibility is orchestration, as mentioned above -- and quite possibly a result of perceptual masking. It can be easy to do this with brass, in particular, because brass instruments have a very "full" spectrum. For this reason they can easily mask other instruments, basically by "chewing up" the spectral content in the listener's ears (okay, so that's a strange way of describing masking, but not too far off! Check out this link for a better description:
http://www.zainea.com/psymusic.htm -- about half-way down the page. You'll notice that this also explains the "violins and trombone" problem.) This phenomenon can be even more tricky when dealing with samples, because there is a sort of acoustical mis-match between the spectral content and the perceived loudness (caused by the normalization process in recording/editing the samples -- two notes, performed in the studio at dramatically different levels, are tweaked into a 0 db peak). In certain cases, you might get a considerable improvement by simply knocking your brass down a dynamic or two, so that it uses a p or mp sample. The lower dynamics have less energy in the high-frequency range (i.e., less "brassy"), and will have less of a tendency to mask out other instruments, particularly those with "thinner" spectral content. If, however, you want the power from the brass, then you could support it with winds... and even _more_ brass!
You can actually check out perceptual masking yourself:
1) make two long notes (tied whole-notes, quarter = 60) in both VI-14 and PO-3 or TP (solo trombone).
2) place the violins above the trombone (a pretty natural decision!)
3) pick a basic "long-note" for the VI-14, and a "pfp" for the TP
4) make the two notes match in dynamic at the _start_ (so that the TP does a cresc/dim "around" the VI-14)
What you'll probably notice is a strange sensation of "dipping" in the violins as the trombone reaches a certain point in its cresc/dim. This is the masking taking effect! Cool... And what's even cooler about it is that this is happening in _your_ ears! (Don't know about you, but I find this pretty fascinating!)
cheers,
J.