You need to spend some time with them. You've become to accustomed to the AX720's sound signature. Give the 880s at least a week ALONE (do not compare). Then go back and see if you still feel the same.
They're not even in the same stratosphere in terms of sound quality and refinement, IMHO.
If you still feel they're not worth it, then stick to the AX720 and be happy. No need to upgarde if you're happy.
I DID specifically mention the 880's aren't the best for positional cues in gaming, especially competitively. People think I'm kidding, it seems. They just do not have the best positional cues.
I will probably keep testing them out for a week or two. Im not lookin too much into positional cues, looking for more immersion but at the same time nothing too bassy. Thats why I chose the DT 880 especially after reading your review.