i never listened to the m20 or the tad300 so i can't give a honest opinion , but i own the superlux hd669 , hd668b and the audio technica ath-t500..so i can tell u about those..the hd669 for me is a decent headphone, it's certainly durable but the comfort is not so much , the pad will get your ear sweating and the grip will crush your skull..but u will get used to it after a while..the sound is good on the hd669 it's clear and the is bass good for the price tag, the noise isolation also good on the hd 669, i would recommend this if u want a close back headphone with cheap price tag ..or if u want a all rounder headphone u can get the superlux hd668b or the HD681 EVO. but those are semi open...if u want something better u can get the audio technica ath-t500 that sell around rm200-rm215