Thinking I need a Sub...

There are good and bad to both solutions.

The larger speakers will allow a more full frequency response that may satisfy your needs. This depends on your room, your system and your preferred lessening level. The dual sub option would more likely meet your lower frequency needs but they may be more difficult to integrate into your system for a coherent balanced sound. You may end up spending even more to get the right software fix for bass integration.

I personally would go for the larger speaker if you like the NOLA sound. Then in a year or whatever, if you are still not satisfied, think about subs again.

No right or wrong here.

I look forward to reading if the OP is able to successfully integrate the sub into his system/room and get a better sound. Its obviously possible to do. I just suspect its going to take a fair amount of tweaking, which some audiophiles enjoy.
But for me, I think that its easier to go to a speaker that is designed to be more full range instead of trying to integrate a sub... if the those speakers are well-designed of course.

Either approach has its positives and negatives. Interesting thread.

I would get a sub or two if I was truly stuck with monitor speakers in my 2 channel system. I do use a Vandersteen sub in my HT (with an audyssey capable proc :)) but that a different ball o wax.