Dirac Live does not integrate multiple subwoofers with each other.
The add-on Dirac Live Bass Control does that as well as phase align the subwoofer group to each pair of speakers and center.
So without having the Bass Control add-on, a MiniDSP is a requirement for mult-sub use.
Align and EQ the sub group using your method of choice and then run Dirac Live calibration. Dirac will then see your sub group as a single sub and calibrate it with the rest of you speakers.
Regarding bass output diminishing: 1) Dirac does its job and removes peaks that shouldn’t be there. Some folks like their peaks (room modes). 2) some folks like more bass than the default Dirac Live target curve. You can use Harman target curves for Dirac with different amounts of bass boost here:
The Harman target curves have no high end rolloff as that should be set individually for your room, speakers, and taste. A good starting point is to follow the natural rolloff of your speakers. The target is adjusted within the Dirac app.
There are other guides and advice in the linked page. I strongly suggest reading and using them. The author has been instrumental in getting Dirac to correct issues he has discovered and documented. Dirac also links to his crossover setting document from within their help pages. So that’s his cred. There’s a lot of other rumor and conjecture out there to be avoided.
Happy to help if you have more questions. There are several ”since it’s inception “ Dirac users on the site.
Welcome to the forum! Happy to have you here!
So I’m just figuring this car stuff out now. Home audio is a piece of cake in comparison, imho