Hi, an opinion for the sake of MX5.
I do not have MX4 Pro so I do not understand your Standard.
I owned MX2, MX3 and MX5 and still keep MX3 and MX5 in my hands.
When I first listened the sound of MX5, I felt the sound is rather soft and rated MX3 is better then MX5 in terms of dynamism.
Now after 2 months since the purchase of MX5, I feel the sound of MX5 has a little bit changed, may be aging (burn-in) is done.
MX5 sounds very quiet but after manipulating the audio max volume in Engineer mode using Meditek tool “Free MTK Engineer Mode”, no more annoyance of being the volume slide is changed automatically by the OS.
Other than 2 Meizu, I use iPhone6s as a Phone because of its global band coverage but both sound much better than Apple.
if Meizu MX? does not sound better than iPhone or Xperia, I have no reason to buy Meizu.
As the result, MX5 is my first music player and 2nd phone while MX3 as the 3rd, just in case of emergency.
I use 3 different earphones,
- Final Audio Design Heaven 4
- Final Audio Design Heaven 6
- Ultimate Ears UE900s
those are dearer than MX5 except 1., but MX5 nicely sounds exhibiting the differences of those characters.