Thank you so much for the insightful advice!
I tried to understand REW previously but struggled with that.
Frankly, my thought is this.
After listen to both, I value mid bass more than low rumble. I am even willing to sacrifice a bit of the low rumble if I can't get them integrated to produce reasonably good results, hence I have option 2. I think the low freq that Dxd-300 can produce will be good enough for me. It also depends on whether there is buyer for my Rythmik. Otherwise, I don't mind keeping them as well.
If I go for option 1, I will try to set up both subs to handle