As I can agree that M500 is clearly better device than DX3 Pro, I cannot agree with some other claims. Don't not why M500 sounded better for you. It has similar power, maybe little less than DX3 pro with 150ohm load. Output impedance of 10ohm is to low to make a difference with HD58x. Maybe it's just placebo maybe your DX3 was broken - who knows. Low output impedance is much more important with portable devices in my opinion...
About Amir's recommendation - DX7s got recommendation (10ohm OI), DX7pro got recommendation (expensive, 6.2ohm OI). DX3 Pro V2 as affordable dac/Amp combo (around $160 on Drop) would probably get it also...
Amir has any IEMs who would have guessed? ;-)
Good thing is you like you M500, that's most important and that what counts.