The Onkyo is close to being a basshead phone. Its bass quantity is above neutral. Its fairly tight bass, but could be tighter, and goes deep. Fairly neutral and clean mids and highs, highs a bit emphasized though. NAD has deep, deep bass and is pretty punchy, very enjoyable, neutral mids, highs a little laid back. M50x has less bass emphasis than the others, although still a little above neutral. Mids and highs pretty neutral. no sibilance but good detail.
Hope this helps. My observations don't coincide with what some others may say. Some say 50x has recessed mids and a strong V-shaped response. I don't find that at all, amped or unamped.
Comfort wise, to me the 50x wins easily. NAD Headband is flat and not well-padded. Onkyo is an on-ear phone but still fairly comfortable, some connector problems have beenn reported. 50x has very well-padded headband and deep comfortable earcups.