Yeah, mixing the music is something I'm still working on as well. So far what I've found to work is writing music while wearing my best headphones (which are only medium-good quality) and I'll get it pretty close, but then I'll listen to it in my car because it has better quality than my headphones to do the fine tweaking.
I definitely don't think your setup is terrible because the piece was very good overall. Unfortunately it can sound perfect on your system and then sound just a little off on another just due to the way electronics are. I agree that listening on more devices will help minimize that issue. Good luck making music!