I think the two that sound nice with Maddox are Aria and Lara. I picked Aria because I prefer the name (it's rarer where I am). Lyra would also go with Maddox, but i'm not a fan of the name because of its similarity to liar. Everyone seems to love Rosalie but I think it's a very different style to Maddox. Delilah and Maddox seem too complex together, It sounds like a mouthful to me. I think less is more when pairing with Maddox, Aria also has 3 syllables but it's simple, yet gorgeous. I agree Hadley fits with Maddox's style but it's too 'symmetrical' for me. The two share the same amount of letters, they're both 2 syllables, and of course theres the 'Had' and 'Mad' problem.
So I vote Aria, goodluck :)