Different systems use all various combinations of speakers, amps and EQs, so different frequencies are emphasized by each. The best thing to do if your having difficulty pinning down the problem, is to check firstly that your monitoring system is up to par (Quality Monitors, placement, room treatment, etc.).
Listen to some favorite recordings on all your systems (Including your monitoring system) side by side with your recordings, and A/B them and write down what sounds out of place to you (Snare louder on system A, too much bass on system B). Then open your files up in your favorite app, and run a frequency analysis or real time analyzer (hopefully you have at least one form of analysis to work with). Compare your notes to what you see onscreen. I tend to shoot for a mix that has a good balance of frequencies across a real time EQ anaylizer (Right in the middle all the way across, but not nescessarilly all frequencies at the same time, that really depends on what the music is doing). you will need to adjust levels and EQs in your mix to achieve this. By the time you get that acclompished, you will also have a more precise picture of what problems you may be experiencing in your monitoring system (I can't stress good monitoring enough!!!). I.E. you may find you have recurring problems with Bass response, which will indicate you need to check your monitor placement, and you may need some form of bass trap.
I personally go even all the way across the EQ spectrum, then adjust the levels into balance from there. Remember that a lot of people use the typical Smile shaped setting on their graphic EQ's when playing back audio on home audio settings. But many home systems may only have 2 bands (Bass and Treble) and those systems are designed (theoretically) to sound good with a flat EQ response, or they might not even allow EQ adjustment (Such as on small portable 2 speaker Tape/CD units...BoomBox style).
Hope this helps. I would definitely look around the web for some ideas with room treatment, Sites like those who actually make the treatments often are a goldmine of information.
Like:
http://www.auralex.com/
http://www.primacoustic.com
