i'm not sure about 47ohms for the m20x however i can say that smartphones handle the 38ohm of the m50s perfectly fine as i have a pair.
if your phone cannot handle it then you could always get a fiio portable amp however for that extra money spent you might as well get a better pair of headphones.
now i cant guarantee that even 38ohm is fine on your smartphone (as specifications across models vary) but i can say that it works fine on my (now older) htc droid incredible 1, ipod classic, asus laptop and many other sources without requiring an amp. volume levels are deafening before i hit the max volume output for the devices as well.
if i had to guess about the 47ohm i would say that you *might* have trouble with some devices but i cant give you a solid yes/no on the matter.
if you live in the usa you can get a pair on ebay for $99 (about double what you would pay for a m20x on amazon) but they are much much nicer overall.
i bought my brother a pair of m30s for xmas this year (again, i own m50s) and there is definitely a significant difference in quality. its very worthwhile to upgrade if its in your budget.