for the m50x to deliver up to 115db (a value high enough to let you listen loudly to even the most dynamic music) you need about 40mw and 1.25v
the X1 goes to 1.5v and gives
Output Power: Over 100 mW (16Ω THD<1%)
Over 65 mW (32Ω THD 1%)
so it's safe to expect still more than 40mw into 38ohm.
all this to say you're clear on both parts. impedance in that headphone doesn't change much so we can rely on those specs @1khz pretty well. it means you will objectively never need more than the X1 for that headphone.
of course I'm only talking electricity here, subjectively some people may find pleasure with more... whatever.
I don't think a sansa is enough for driving this headphone well when listening loudly(it's 7.5mw into 32ohm, or so said nwavguy). it should still be fine for moderate volumes as you should reach around 105db peaks.
sorry if I take the fun out of audio a little.