Digital is digital..... basically if you take the mix from one computer to another the mix doesn't change. It will be the same exact mix from the original DAW to any other computer and even to a CD that you would burn.... the mix will be the same.
Therefore..... you have to look to see what HAS changed from one "mix" to the next and the answer is:
1 the sound card
2 the speakers
3 the environment
In the OP #1 you said:
I know it's not the monitors, because I've used them before with consoles in the same room and never had this issue. Plus, I connected different speakers (from system B) and got the same sound. So the way I read that is using the same speakers in the same room on a different system the mix sounded exactly the same. But, when you move to a different room or environment, throw in some different speakers, it does not sound the same.
This is a common problem that is related to the original mix and how it was done. This is the reason they sell acoustical room treatments and hardware as well as software which is able to analyze the given mixing room and attempt to correct the deficiencies it detects in the room to give you as close to a flat response as is possible given the original acoustical problems in the room.
If you can't hear something, you can't mix it correctly. Basically if you are mixing on headphones or stereo speakers or even the inexpensive "studio monitors" sold in the pro-audio depts of music stores, and doing this in a spare bedroom, basement, or corner of the living room, there is no way you are hearing the TRUE acoustics of the music. Between the acoustically bad room, and the inefficient and biased sound reproduction, it's at best a hit or miss proposition to get an accurate mix.
There is no easy "do this or that" answer to it. A combination of try to fix the acoustics, get better gear if you are using home stereo or headphones, look into the room correction software solutions, and most important, learn the strengths and weaknesses of the system you are using.
Compare the pro mixes on the commercial CD's and use that as a starting point. But, in the end it's about learning and listening. Fix what can be fixed and learn to compensate for what can't.
You can produce some amazing quality in the corner of a spare room, but it will take some work.