Well if the sub is new, I would return it and invest in something better. That will ease the calibration process, and if you did get new subs with auto cal normally you’d run it in the subs first, and then audyssey. Then, what many do, is use the audyssey app to allow audyssey to only use correction below about 300hz. Or use LR bypass.
No you don’t need a minidsp. Yet...lol