And I would bet my left nut that it's not the "superior mix bus within Pro Tools" either.
Question #1: What audio interface did you use before the MBox?
Question #2: Can you use the MBox with Sonar? (I don't know if this is possible that's why I am asking).
Question #3: If the answer to Question #2 is "Yes", then try to record the same exact thing through the same interface into both Sonar and PT and see if there is a difference.
1. I'm not using the Mbox, and that wasn't the basis for my opinion (though I do use one occasionally) it was the 003. I wasn't using the 003 with Cubase, but I have gone back and recorded in Cubase with it, and I still don't think it compares.
2. Yes you can use an Mbox (or any Digi interface) with any application which supports Core Audio (Mac) or ASIO/Wave Drivers (PC)
3. I don't have Sonar, but the OP does and I agree he should try that.
To start:
http://forum.cockos.com/showthread.php?t=13473
If you have any questions, PM pipeline he posts here also.
Another thread describing almost the exact same issue you are having(this is a very common question, and the answer is almost always, the difference in pan laws between DAWS)
http://www.gearslutz.com/board/music-computers/380486-cubase-no-match-pt-level-playing-field.html
I know what the Pan-Law is, how it applies etc. That's not my issue.
I am dismissive of that because I can create mono mixes which sound better in Pro Tools* than in Cubase.
Whatever the result, I'm not actually bothered. I am happy using whatever the heck I want to. However there are far too many people around here who are so quick to shoot someone elses opinion down, and return it with insults, general rudeness and in a condescending manner.
* - This is my opinion