I had the chance to audit the XB500, and I have a pair of M50S.
Technically, depending on what kind of bass you want, they are both about equal. XB500 has more low bass extension, so the phones rumble more. M50S has more mid bass and upper bass emphasis, so it doesn't rumble as much, but the impact is definitely harder with my M50S.
I wouldn't say the XB500 is muffled, but it definitely doesn't have as much extension in high frequency as the M50, which can sometimes sound too harsh depending on the recording.
What I didn't like about the XB500 was that the soundstage was not as open. But for roughly half the price of the M50, I think the XB500 is a nice choice for anyone under a budget.
But if you can stretch your budget, M50 + a good head amp with bass boost does wondrous to the sound quality. With the XB500, it's very hard to boost the bass because things get overpowering very quickly.
Edit: also forgot to note the important part: I know there are at least two kinds of M50 sound signature. One is a V-shaped response where both high and low frequencies are boosted, giving more bass and treble extension but recessed mid. The other is a flatter response, giving less bass impact, less treble sibilance, but overall more balanced. And from my experience, it may depend on how you burn in the headphones, or... other factors. But I know that this difference exists, as my dad's M50 (coiled cord) sounds very different compared to my M50S (straight cord).
My M50S is the flatter response.