Well - if you want to let your stuff sound good on several systems, it is not enough to listen to one set of speakers only. You have to indeed listen in your car, on your stereo, on (if possible) a small transistor radio etc. and make notices what does not sound "right". Then you go back and make corrections and try again. That's the way it works.
But I guess you make a fundamental mistake when you are mixing on a pair of speakers and just mix until its sounds "right" (that is - only the most pleasing). If you do it that way, you are only make a piece of music that plays right on your MA-15D set, bot not on anything else. Keep in mind different systems have different dynamics, frequency spectrum's etc. Also - the environment (closed car, cellar, living room, hall) have different responses that boost or dim certain frequency's.
What you should do is listen to reference material first. Take a song/piece of music that sound right in your car, stereo etc. and then take a good listening how this song/piece of music sounds on those MA-15D. Eventually make notices how the balance and spectrum sounds on that system (like "gaps" or "boosts" in the frequency spectrum). Do not use one single piece of music, but compare several pieces to get a global impression.
Now - with that reference in mind do your mix again, and try to "home in" to the balance and frequency spectrum you heard in the reference material. After that you still have to try out how this "works" on other systems, but you will see you have much less work to do.