The EX71's are my favorite headphones, and i have had alot in my life. One pair of every ipod headphone (2 of the 5th gen), tons of sony's, logitech, koss, senheiser, phillips, bose, you name it, and most likely i have had a pair by that brand. the EX71's reduce noise because of their in-ear design and silicon earbuds. they have a great frequencey range of 6-23k and really bring out the best in your music. if i could afford the $500 fancy overpriced over-rated, i still wouldnt get them, hope this helps.
mdrex71sl earbuds are good but have a bit too much in the 4-8k range. mdrq33lp are great headphones but don't expect earth shattering bass. i actually prefer them over the ex71 because they sound much more neutral. ksc75 is better than ksc50 and costs the same on amazon.com. ksc75 and the 33lp sound similar it depends on other factors after that point. like would you prefer koss over sony, is the sensitivity important, etc.