[T. Greig]

Slaveern is right on with this one. To preceive the frequency spectrum as flat as possible, you should be mixing between 85 to 95 dBspl. Mixing at anything below that your precieving higher midrange while your lows and highs are rolled off. This is why your snare sound gets boomy when you turn up the mix. Your ears are hearing more lows and highs. You ears will become fatigued quicker than when mixing at a lower level, but you'll be much happier with the sound of the end result if you mix hotter. As far as this translating to other systems, I would look to see if your cutting lows and highs out of the system. If you have been, then that would explain why the system sounds flatter.