Dvaitadvaita gives the same explanation of Nirguna that Visistadvaita and Achintya Bhedabheda give. Here's what Roma Bose says in this excerpt from her book "Doctrines of Nimbarka and his Followers":
It is clear from the above that Brahman is essentially possessed of attributes. On the one hand, he possesses an infinite number of auspicious qualities, ... and on the other hand, He is absolutely devoid of all that is inauspicious, unworthy and defective. In this sense, the Lord is possessed of a double set of characteristics, one positive, the other negative. And it is the negative aspect of the Lord, i.e. His freedom from all unworthy qualities, which sometimes leads to the description of Him as attributeless. He is thus attributeless in the sense of being free from all ordinary material and inauspicious qualities, and He is possessed of all celestial and auspicious ones.
Dvaita also gives the same explanation. Here is what B.N.K. Sharma says in this excerpt from his book "A History of the Dvaita School of Vedanta and its Literature":
Texts which predicate certain auspicious attributes of the Brahman cannot put up with any but their literal sense (niravakasha) whereas the Nirguna could be accommodated in the sense of denying material attributes and so find their fulfillment of purpose (charitartha). Vishnudasa proceeds to show that there is no irreconcilable conflict between texts like "yas sarvajnah" and the so-called 'Nirguna text' "Kevalo Nirgunashcha".
By the way, the Dvaita philosopher Vishnudasa is using a Purva Mimamsa principle called Savakasha-Niravakasha Nyaya. It states that if two texts seem to be in apparent conflict, one should resolve the conflict in such a way that both texts have scope.